Well I built the system quite a while ago - probably almost 2 years ago now. It's a q6600 (not overclocked) and a factory overclocked XFX 7900gs, on a gigabyte p35-ds3r motherboard. The psu is a 600watt Zalman. 4 x 1 gigs of corsair ram.
So I left my pc on overnight downloading stuff, so this morning I move the mouse to wake it up. Nada. Orange light on the screen indicating that the screen is in power saving mode. I can't wake the pc up via the mouse or keyboard. Ok, so some kind of Windows crash (running XP w/sp3) I think. Hit the reset button. PC resets and boots back up. I log in, browse the net a bit, and then suddenly the monitor goes into power saving mode, presumably because there is no longer a signal going to it. I move the mouse, do ctrl+alt+del etc. Nada. Ok, ***, something else is wrong here. I reboot, go into bios, check the system health. Everything seems ok - system voltages are reported as OK. But then the same thing happens in the bios - no signal to the monitor. Darn, so it's not just some kind of software problem or virus by the looks of it.
So I shut it down and think ok what could it be? Maybe one of the rails of the power supply is dodgy? So I change the card onto another rail. Reboot. Log in. PC seems to be running ok, and keeps doing so for about twenty minutes. So I think 'Ok so maybe it was the rail on the psu'. Walk out, go make some coffee n ***. Come back. As I walk in the room, same thing happens - no signal to monitor. This time I was able to hit Alt F4 and U to shut the system down normally. Reboot. Windows boots and runs for a few minutes, whereupon the same problem occurs. This time I couldn't alt f4 or anything.
Ok, so I remove the ram and try it with one stick. Same crash after boot. Try another. Same thing.
I really don't understand this problem at all, because the machine runs as you would expect it to - 3d programs run - temperatures are ok, but after a couple minutes there's no signal to the monitor and the pc becomes non responsive. I also tried reseating the graphics card to no effect.
Oh, and two details I should mention. In the last weeks occassionally my mouse would shut off, though unplugging it and plugging it in again fixed the problem - I put that down to just a bad connection as I use a usb extension cable.
The other detail is that Gigabyte's volate monitoring software says that the motherboard's +12v voltage sits at around 12.8v, which is a bit high, but it has been reported at that level ever since i put the machine together as far as I can recall. Also software tools are known to report voltages incorrectly. The bios itself says the 12v voltage is 'OK' though it doesn't report a number.
Sorry for the wall of text. I just wanted to be precise.
I'm wondering if there could be a charge building up somewhere, which might explain the reason it takes a certain amount of time before the system locks up. Seems the problem could be in the PSU, the mobo, or the graphics card.
Sadly I don't have any of these spare that I can swap out at the moment. Maybe I'll see if I can borrow a few.