I just built a new computer. The first time I ran it for about an hour, then it shut off. I thought it was overheating, so I let it sit. It turns off immediately now (0.5 - 3 seconds after powering on), even after letting it sit for hours. I've tried re-applying Arctic Silver, but it didn't help at all. Weird thing: It sat for about a week without use; I tried the computer again today, and it ran for about 30 minutes before shutting off.
I removed everything. I put the motherboard on a static-free bag on my table with only the processor, heat sink/fan, and PSU. It still shuts off immediately.
Motherboard: XFX nForce 790i Ultra
Processor: Intel Core 2 Quad 9300
PSU: 1200w Coolmax
One last thing: In its current state (on my kitchen table), computer does not experience this problem if I leave the 8-pin cpu power connector unplugged.