I am having a very strange issue with my computer. If I start it 'cold' (meaning it's been off more than 3-4 hours), the computer will boot up, allow me to login to windows, function completely normally, and then turn itself off suddenly after 5-10 minutes. However, it doesn't completely turn off; I can still hear fans running within the case, but there's no output to the screen, and no power to the peripherals (aka the mouse and keyboard act as if the computer is turned off). Sometimes when this happens, I can turn it all the way off simply by holding the power button, other times I have to manually flip the switch on the PSU. The really bizarre thing is that after this happens, and I turn the computer back on, it runs fine. I have literally left it on for 3-4 days, and it's functioned fine. But then if I turn it off and turn it back on a few hours later.... same problem. I am posting it here b/c it seems to me like problem this bizarre has got to be mobo-related.

Specs:

MSI P6N Platinum SLI
Intel Core 2 Quad 6600
PC Power and Cooling Silencer 750
4 x 1 gig Crucial Ballistix PC2 6400
BFG Tech BFGR88640GTSOC2E GeForce 8800GTS 640MB
Cooler Master Cosmos 1000 Case.
Running Windows XP Pro 64 bit.

Initially, I suspected the PSU. I rma'd it back to Newegg, got a brand new one back, same prob. I've run memtest to make sure the memory is ok, no errors. I've run 3dmark to stress the graphics card, it worked great. Run superPI on all 4 cores to stress the CPU, it held up. I tried to RMA the board to MSI, they "tested" it and sent back the same one, saying it was fine. I just can't see a problem this bizarre being anything but a mobo issue. Anyone have any advice, otherwise I guess I'm just going to buy a replacement (non-MSI this time) mobo.

BIOS will have wake on USB event or keyboard/mouse... make sure all that junk is turned off..

Make sure that all power saving in windows is disabled, hibernate and sleep etc.