Computer seems to randomly 'sleep'

Hello everyone,

I seem to have a reoccurring problem that I thought I fixed when I bought a new power supply which fixed the problem I was having with not being able to get my computer to stay on after the boot process. This particular problem has happened multiple times but occurs only every so often though it continues to happen for hours after it has started.

At any point in time my computer will randomly turn black and the CPU and GPU will shut down and all fans will come to a stop. I don't think the hard drive shuts off because I am able to turn the computer back on (returns to desktop or last open program) with the keyboard.

After the problem happens and I turn the computer back on I immediately open HWMonitor and all the temperatures are reading ok every time. Recently I have been able to make the problem happen right away by pressing keys on my keyboard (ctr+shift seemed to do it) but it happened when I pressed other keys too. Once I noticed this I let the computer run without my doing anything and it did not sleep by itself.

My system specs are:
AMD Phenom X3 720 BE (OC'ed to 3.4 without added power)
Gigabyte UD4P Mobo
G.Skill 2x2 gb
Thermaltake 750W (recently purchased)
Radeon Sapphire HD 4870

As I said, I replaced my PSU and also reapplied thermal glue to the CPU heatsink. What could be causing this problem? Should I consider reinstalling my OS? Could my keyboard be the issue? I was able to notice the keyboard causing the random sleep this last time and could imagine it was the root of the other happening because I was active on the computer every time it has happened. I have updated graphics drivers (CCC 10.7) and windows 7 is up to date. Norton finds nothing on my computer either. Any suggestions?