Random shutdowns without errors

I'm in a quandary here: I have a PC running Win 7 Home Premium, AMD 64 x2 6000+, ASUS mobo M2N-MX, AMI BIOS v0406, 4G DDR2, 550w PSU, XFX 9600GT video card, 2 150gb SATA hard drives. My wife only uses this for email, internet, and the occasional Word document.

Her PC has begun shutting down hard randomly. It first occured with a variable number of days between every reboot. I opened the case, made sure everything was dust free, reset all the PSU, SATA, CPU, and video cables. I added a CPU temp monitor software program, thinking that the CPU was overheating, but it stays between 96 and 110 degrees on each core.

I thought that fixed it but, with 2 1/2 weeks since the last reboot, it now shut down again. There are no error messages that I can find in any of Windows event monitors except to say that Windows did not shut down properly.

I ran checks on the HDDs - all OK. I ran Windows memory diagnostics - extended testing - and everything was OK with memory (I think if memory was bad, I'd have a BSOD). I restarted the PC with a Ubuntu ISO and LINUX loaded fine.

I would also think that if this was a PSU issue, there would be hard failures, not random shutdowns. Also, if it was a capacitor issue on the mobo, the times between shutdowns would lessen not increase.

Does anyone out there have any thoughts on this?

Let's try the old fashioned temperature monitoring device - your finger.
Wait until the system shuts down, then unplug the system and open the case.
Carefully run your finger around inside the case - don't touch anything, just get near it.
If anything is excessively hot, you'll feel it (be careful not to get burned).

Have you checked to see if there are any applicable BIOS updates? My son's M2 board needed a flash to support his nVidia 8800 video card.

I looked at the ASUS site and there are 5 BIOS releases since the pc was built. I will try to download the newest tomorrow and see if I can get it to apply. Prior to that, I will do the 'finger' test but I want to see if the PC will runn the full 24 hours.

Thanks for the quick response. I'll get back to you tomorrow.

If there's 5 BIOS releases, only do it in steps of 2 or 3.
For example, update 2 versions, restart and make sure everything works, then update 3 versions and test again.