Trouble shooting help

I was wondering if someone might be able to give a suggestion. I'm having a very intermitent lproblem, and need some ideas.
This is my problem:
My computer will be running fine for quite a while but within 24 hours it will instantly power off, no error, no warning. I've already replaced the power supply thinking it was that. I've pulled and reconnected all the connections. CPU heat is around 45 degrees celcius. I have to hold the power button for it to try and power back on, and usually pukes again right after windows boots. If I leave it unplugged for a while it's fine. None of the components, asides from the hard drives feel hot. Everything is cooler then the underside of my pillow. I'm at a loss, it's a fairly new system.

This is my hardware:
Asus Crosshair
amd dualcore 5400
2 gigs of corsair memory
700w thermaltake power supply / Had a 520 corsair before
and a max orb cpu fan

I'm really at a loss as to what to try next or what to check. Any help would be much appreciated.

OK first off you may actually have downgraded your PSU (but that is another story ).

Have you run the usual engineering tests. I mean poking about on the heatsinks and feeling for warms spots with your fingers!! Thats what I usually do when I have a problem. A lot of people just think about cooling their CPU and focus on this. What about the RAM temps, the chipset, the MOSFets for generating the CPU core voltage from the 12V rail, etc.?

The best way to test for thermal instability is to take the side off the case and place a 9"/12" house fan beside the rig. If the rig can pass FFT's, CPU burn in, and other stability testing apps. etc. when it couldn't before then you have isolated the fact that something is overheating.

What case are you running? One of the problems with the "orb" type coolers is that it sprays heated air out all over the MB!! If you aren't careful it will also recirculate hot air in the case. (Why I prefer to use watercooling but that is another story - ahhh got to stop using that phrase... )


Bob