I have an interesting problem and many days later I think i narrowed it down to a bad motherboard. What was happening about 8 weeks after i moved into a new house which later I found out after the problem the whole area is known for dirty power. before you all get on me, I just recently bought an APC, but not till after the fact.
The system would turn click off shortly after i hit the power button like it was a bad power supply, NP I replaced the 1K PSU with an 850K PSU. The system would turn on and after its been on for awhile it would click off. I monitored the temp of the CPU and saw it was running hot, around 60 C. I have had the system for 5 years now so I took the heat seek off and put new thermal paste on, temp runs about 10-15 C cooler now. i took out one video card and tried one at a time and eventually the system would turn off no matter which PCI-E slot it was in. Then I tried to stress the video card some, I ran the ratings test that is on Windows 7 and every time when it starts testing around the direct X stuff the motherboard clicks off. Before I was running XP Pro - 64bit and upgraded to Windows 7 ultimate and same problem with clicking off.
I am thinking maybe the motherboard took a power spike since it was only plugged into a power strip and now the PCI-E slots don't work correctly ?
I replaced the video cards with one nvidia Geforce GTX 550 Ti 1TB , only got one not trying to run SLI right now