I have had 2 instances of a problem similar to this. My motherboard is an ASUS M2N SLI Deluxe.
The first was when my motherboard went out on me when I plugged the front USB into the firewire slot. Not good. I would hit the power button, it would turn lights/fans on, then shut off within seconds.
The second I have just started to experience. My computer recently started freezing, and it appears to be the hard drive. I have run numerous tests, and they say my hard drvie is ok. I am convinced its not, since it is constantly freezing during boot up. When I perform a cold boot, the OS will freeze, making me reset the computer. When I do this, the computer will shut off twice before it will finally go through a good clean start.
First, I would knock the OC back to stock, even though it seems perfectly fine. Then, my first to places to check would be the motherboard and hard drive.