Tarkus11 

You should look under your Control Panel/System & Security/Administrative Tools/Event Logs to see if Windows registers an error in the log. If there is a Crash Dump file you can get a general idea by looking at the description after installing this WhoCrashed - CNET Download.com As ugly as it sounds, you may need to boot up in Safe Mode with Networking and run all your apps to try and crash it. If it doesn't crash, you have narrowed the problem down to something loading during StartUp.

No dumps or logs - but I will try safe mode. Thanks for that!

DionysusToast 

BTW - silly question - does video rendering use the graphics cards? I would presume it's all CPU because it's not actually outputting anything to screen.

Not a silly question at all.

Most modern graphics cards have more or less RAM on board (depending on the model) and use their own (GPU) processor to do video rendering tasks since the onboard CPU of graphics cards is much more powerful and optimized to master such (CPU intensive) tasks.

So, perhaps there could be some parameter settings on your graphics card software which could cause the problem....

You should turn off the auto-restart feature so you will get the BSOD to see what the error actually is.

To turn it off:

Right click on 'My Computer' select 'Properties'
Select 'Advanced system settings' then click 'Settings' button under 'Startup and Recovery'
uncheck the box 'Automatically restart'

DionysusToast 

Mike - it does one of two things... 1 - Just stop - all screens still displaying but frozen, mouse & keyboard unresponsive. 2 - Reboots with no warning and no BSOD - just resets itself I'd love it to BSOD on me one time but a freeze or a reboot is the best I get and there's never anything in the event logs that occurred prior to the reset. I'll run the memory test & try the fan thing too - thanks for the tips.

Judging from this description, I'll risk it a little and suspect it's your hard disk drive above everything. Have you been using an SSD for over 1 year, e.g. Crucial m4? My next suspect is your power supply. What is the power capacity rating of your power supply? Did you just install any new parts whatosever? How long have you had the power supply and how long do you leave your PC on per day? It's very possible that the capacitors in the power supply have aged if you leave it on 24/7 - and if there isn't excess capacity in your power supply, you could run short of power. This usually leads to spontaneous resets. This doesn't sound convincing without an example: I have a 550W power supply with nearly 540W of components plugged in (overclocked), and give it a year of 24/7 usage, I would already start expecting crashes from insufficient power. Use a calculator like this: https://outervision.com/power-supply-calculator to make sure that you are in the safe zone. After that I'll suspect your memory.

A BSOD error code would give many telltale signs, so follow what the previous posters had said.

Assuming the PC was assembled carefully (especially with thermal paste on the CPU), it is practically impossible to overheat with those specs. I will rule out the processor cutting off the power because of temperature. The Intel Sandy Bridge chips should have a T-junction temperature at 98 degrees celsius, practically impossible for you to reach no matter what you do and wherever you are. You have a great motherboard that can take much more beating, so it's unlikely to be there either.

If you highly suspect it's the graphics card (I'll say it's unlikely from that description) and you're not too afraid to do something...

1. Get a suitable Philips head screwdriver, remove whatever VGA/DVI/HDMI cables going to your graphics card.
2. Touch some metal surface before you do this, e.g. your PC case, to prevent electrical discharge
3. Unclip your graphics card the PCI bus
4. Unscrew the 2 screws that hold it to the back of your case
5. Gently pull out your graphics card (it is a snug fit, so you may need to push it gently out from behind) and leave it say on the inner surface of the case cover that you just removed.

Since you have a Z68 motherboard, it should support your processor's integrated graphics chip - now plug your main display cable onto it (you should see a similar place to plug it higher up from where the graphics card's backplate was) and now you should be good to go - switch on your PC and see how it goes. If it doesn't crash in the same way again, it's your graphics card at fault.

TL;DR: Hard disk, power supply and memory are usually less robust. I'd put the motherboard somewhere right after this or next to the memory, but your motherboard sounds rather new and early for problems like this.

If you've had the problem since you built it then make sure you are running the latest BIOS (first) and firmware (second) and drivers (last).

Lots of fantastic advice here many thanks.

So- I was looking set to spend a few days over the holidays swapping parts in & out of the PC. Last night (yes, Chirstmas day), I was on the PC downloading some Android games for my daughters new tablet.

So - I have open the Google play website and my email client. Nothing else at all.

Then it crashed & actually gave me a blue screen (presumably because I took the advice on here to stop the auto-restart).

The only thing that came up on the BSOD was that it had created a mini dump and to look on line for MEMORY_MANAGEMENT

I also looked at the event long and that's confusing me more.

CrashEvents.xlsx

It seems to start off at 11:18 with some file structure issues
Then at 11:21 complains the gprahics subsystems are overloaded (must be those 2 apls)
Then at 11:23 a bunch of programs appear to crasg

None of this I noticed at all - the PC didn't crash till just after 11:30

So - it's complaining about disk, graphics & memory? That doesn't make much sense unless of course it's the power supply at fault I guess.

Anyway -plan is still to remove & test one by one.

@DionysusToast

I have been making and breaking and fixing computers since the early 90's so here is my 2 cents

Based on your symptoms I would go with your Hard Drive or your graphics card as the two possible culprits. The power supply would be a close third.

Each of them will cause part of the symptoms you are describing. But to me it sounds like the hard drive.

A graphics card can cause this issue but you will generally get a frozen computer screen with weird graphics on the screen at some point. Kind of old school block graphics. If you have not seen this then start with the hard drive.

A power supply failing can cause this issue as well, but it acts a bit different when it fails. Generally you do not get a frozen screen. Instead you will get a hard reboot for no apparent reason. Eventually when you go to start the computer the fans will spin for a brief second then nothing will happen. At that point you know its the power supply!

If your CPU overheats your motherboard should start beeping at you and will shut down. Also you will generally get a warning that the CPU is overheating from windows. If you installed the monitoring software which came with your ASUS board then you will get warning from the software. If your CPU is overheating you will also hear your fans inside your box going into overdrive. It will sound like bathroom fan running inside your computer. This is sure sign that your CPU is overheating.

If the inside of your box is covered with dust then clean it out. This actually prevents a lot of problems from occurring due to heat. If you have the stock CPU fan which came with your i7 then those things get clogged up with dust easily. Good way to tell: If all you see is gray when looking through the CPU fan then it needs to be cleaned. You should see silver with black spaces between the fins.

The hard drive causes all the symptoms you are describing which is why I would start there first. This is also the hardest to diagnose because the error codes point to something else.

Finally, memory can go bad but it does not happen often. Most of the time memory will go bad within a short time period after installation. If you have had it more than a year then I would discount memory all together as the problem. This is not to say it could not be the problem, just saying the odds are slim.

Hope this helps!!

Robert

Hi Toast, Looking at your log, I'm not sure either if it's hardware or software. It's possible your OS may have been damaged, especially if you had BSOD and system shutdowns ever since you first started using your setup. Have you tried this?

Run the command prompt in administrator mode. (from accessories, right click to get to the "Run as administrator" checkbox option)

Then at the virtual dos prompt type : chkdsk /f /r
Then restart windows and it should do a system and file system check and fix if needed

Also if you have your original windows install disc, you could also type in dos prompt type :
sfc.exe /scannow

which will compare your system files to the originals in the system store and the install cd(dvd) if needed.
(warning: this will re-write your system files to un-updated versions. mainly to fix damaged system files if there is no other recourse.).

Another tool to check your drive can be Western Digital's diagnostic. It can do a full disk sector scan without overwriting the drive. ( needless to say don't choose write zeroes!)

If the hard drive is ruled out, maybe check your processor fan and heatsink if not done so already? Accumulated dust can easily slow or stop the fan overheating the cpu quick. And of course the paste as mentioned.

if you have a spare hard drive, you could also swap out your system and basically install a new fresh Windows system to see if you still have the same bluescreen problems after some use to help rule out the hard drive and OS to lean to more of a hardware issue.

Good luck.

Went through something similar with my off-the-self HP h8-1221 but could pinpoint the ATI video driver/ AMD Radeon HD 7570 as the culprit, symptom being random BSOD.

Issue vanished after an operating system update (and removing a cigarette butt from the card, which could have been mine but if so no clue how it got there; maybe a bit of wiggling cards that were already apparently seated properly).

Thanks again, all.

Based on the comments so far, here's the plan.

I purchased a set of burn-in test software (BurnInTest by Passmark) & ran it in "test all" mode overnight to be welcomed by a BSOD NTFS issue. I rebooted & looked @ the log in the burn in software and nothing logged!

I followed this with individual burn in tests of both memory and then hard drives - for 6 hours each and neither failed.

I will continue through all burn in tests individually in the hope that one fails repetitively for the same reason.

The guy that built the system will loan me a new power supply to test that, which will be in January.

Operating system is windows 8,. upgraded from win 7. I'm going to leave the OS in the short term and I will consider a BIOS upgrade after the burn in tests if they yield no additional info.

I will also rip out the video cards and use the on-board intel video to run the PC & perform the tests again.