PC Turns on then Instantly Shuts Off

Hey all,

I've been having a very strange problem lately: When I press the power button on my PC, the fans spin, the lights turn on, then half a second later it shuts off. About 5 seconds later, this process repeats, until I flick the PSU switch.

Recently I moved my PC from one room to another. After I moved it, I didn't use it for about a week or so. When I tried to turn it on after the week, I started getting this problem.

I have an Asus P6T, a Core i7 920, a Radeon HD 4670, a 650W Ultra PSU, and 6GB of DDR3 RAM.
I've gone through a ton of steps to troubleshoot:

1. I've removed the motherboard from the case and installed it on a piece of cardboard.
2. I've tested the computer without the RAM, harddrive, DVD drives, etc. Only the CPU. I still get the same result.
3. I've used a different PSU.
4. I've tried different power outlets.
5. I've reseated the video card, the CPU, and the RAM.

Nothing seems to be working!
Any help is appreciated.

Thanks.