It can go for 2 days without incident or have it happen 2-3 times within hours.
Here's the info I have gathered so far on it:
I have recently upgraded my GFX card from a GTS250 to a GTX680 and had about a week of using the computer without a problem. Lately it seems to suddenly shut completely off (as if you pull the plug) without warning and start up again a few seconds later.
It does not seem to be a temperature issue, since it sometimes happens when the PC is idling and so far it has never happened when I'm rendering 3D stuff for hours. Speedfan does not report any unusually high temperatures either, whether idling or stressed.
The PC is also plugged in a UPS (Uninterruptible power supply) but even when plugged directly to the wall plug, it still seems to shut down.
The randomness is obviously the biggest issue because it is hard to see if something I change actually did anything. Few days ago I thought I found a trustworthy trigger for these shut downs - every time I scanned my OS SSD drive with avast it shut down the computer within a minute. Happened 4 times in a row so I shut it down, cleaned up all the dust etc from inside, unplugged and replugged everything and rebooted.
Scanned it twice and got no shutdown, I crossed fingers that this was it, but sure enough the day after, while playing Skyrim, it shut down.
PSU Chieftech 750watt ~3 years old
MoBo Gigabyte Z68X-UD4-B3 ~7months old
CPU Intel i7 2600k OC 4.3GHz ~7months old ~5months OCed
CPUCooler Corsair H60 ~7months old
RAM Corsair Vengeance LP 4x4GB 1600 ~7months old
OS SSD OCZ Vertex 3 120gb Firmware 2.19 ~7months old ~3months with 2.19 firmware
HDD 1 Seagate 1TB Barracuda 7200 ~3years old
HDD 2 Seagate 1TB Barracuda 7200 ~2years old
With all of that new stuff on a 3 yr old craptacular PSU. You cannot, I repeat CANNOT skimp on a power supply when you are powering your new hardware. If that goes out it can take out your entire system. And youll be SOL cept for the RMA of the power supply, cause they wont cover your system. Thats my 2 cents.
Its always possible, but even if it is bad, you need to replace that PSU with a good quality XFX, Corsair, Seasonic or Antec EarthWatts. If you are in doubt about a PSU at any time check out johnnyguru.com for PSU reviews, and lots of them.