PSU or MB error?

[MattShinners]

I built a new system about three weeks ago. Randomly (at least it appears that way), the system seems to go into a 'sleep' that it can't wake up from. After leaving it idling for awhile, the case fans and lights turn off. If I click a mouse button, the system stirs for a second (case light on, fan on), and then goes to sleep. It then cycles through this until I turn the switch on my PSU off. After that, the case fan and light goes on again for a second, stops when it doesn't have any power, and turns off. If I turn the PSU switch back on, it lets me boot up as normal.

I'm thinking it's a problem with either the PSU or the motherboard:
MB: GIGABYTE GA-X48-DQ6 LGA 775 Intel X48 ATX Intel Motherboard
PSU: CORSAIR CMPSU-750TX 750W ATX12V / EPS12V SLI Ready CrossFire Ready 80 PLUS Certified Active PFC Power Supply
RealTemp log shows that my core CPU (Q9450) didn't go above 40C during the time that it was on, and ATI CCC shows my 4870 at a fairly constant 45-48C. I know that the Gigabyte MB has a weird energy saving mode that I'm not sure if it's on or off, or if that could be what's causing the problem.

Any ideas? Should I find a new PSU and try to switch it out? Or does anyone know what could be causing this?

If you've got any ideas or need to know more info about the system, let me know.

Thanks,
Matt

(Other system info: running Vista with 4GB Mushkin RAM, Seagate 320GB Barracuda, LiteON generic DVD/CD drive in an NZXT case)

 

[nutbag]

Did you update your bios? I had a weird power issue with the EP43-DS3L. I swapped out the PSU just to make sure. It was the bios. My board wouldn't power off. I thought it weird that a new board would have issues like that, so I tried everything except the bios update. I even used the @bios utility to do it. Worked like a charm. Also I think the energy saving utility is software I had to install in the OS. Check the bios for other options.