Automatic shutdowns and reboots. Can't stay on for more than 15 minute

Recently, my PC has been shutting itself down (as in powering down) and rebooting seconds later. This started out at random times, both in use and completely idle, but it's got progressively more frequent to the point where my PC can now no longer stay on for more than 10 minutes, sometimes shutting off before I even get a chance to log in. I've been intensively and ultimately hopelessly troubleshooting for days now. Here's a list of what I have discovered:

I've replaced my PSU.
I've checked for overheating and even replaced the CPU cooler.
I've run memtest86 and all tests were passed.
I've unplugged all nonessential hardware.
I've plugged the PC into a different power socket.
I've run DSKCHK.
I attempted to run two virus scanners from booting them from CDs, but my PC shut down, although it was fine for the memtest, and stayed on for an hour in the BIOS settings without shutting off.
I used the command prompt on the Windows 7 disk to scan for errors. None found.
I've switched out my mouse and keyboard.

Despite all of the above, the problem still persists. However, my system APPEARS to run fine in safe mode.

Anybody got any thoughts at all?

Pauly said:

My initial thoughts would be overheating, I know you have changed your cooler but have you actually monitored temperatures from power up to a reboot to see if they are progressively rising ?
If it is an overheating issue then you would expect power up time to be at its best following a cold boot decreasing after reboot as machine is already warm

Check event viewer and see if you can get any clues as to why the machine is shutting, check for errors immediately prior to a shutdown

Sorry, yeah I've made sure to monitor temperatures and my CPU and GPU never exceed 50 degrees Celcius. They have been known to get into the low 80s while gaming, but my PC never shuts down during gaming since I've not even had the chance.

Pauly said:

Check event viewer see what you can find

Is the machine performing a controlled shutdown or is it hard crashing to a reboot

Last time I checked event viewer all I could find was the usual 'the last shutdown at x time was unexpected'. I'll check it again when I next get the chance and post my findings.

As for the rebooting, the PC is literally powering off as if the power has been pulled, and powering back on again seconds later.

Another thing I forgot to mention was that I have disabled automatic restarts when fatal errors occur, but it still does this.

whs said:

For a test, run a live Linux CD for a while. If that stays up, we know the hardware is not at fault and we narrowed it down to a Windows problem. If Linux does not stay up either, then it would be a hardware problem.

If you have no Linux handy, make yourself a little kit like this. Then boot from the USB.

Emergency Kit - save your files from a dead OS

PS - that kit may be handy at other times too.

Thanks, I'll try this when I can. If it helps however, I already tried using Kaspersky Rescue disk, and it shut down during use of that. However it didn't shut down during memtest86, which was purely text-based, and I left the BIOS settings open for an hour without a shutdown.

whs said:

So you have actually 2 different test results. The Kaspersky would suggest a hardware problem and the memtest would suggest a software problem.

I think you need a third opinion.

Well safe mode seems to run fine, which is what's been baffling me.