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?

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.