Desktop shutdown after sleep

I've been experiencing shut downs in games or really any graphic intensive programs about 10 minutes after my computer wakes up from sleep (not hibernate). I bring up the sleep because in the past I would experience choppy performance from games like half second skips in frame-rate until putting the computer to sleep and then waking it back up after which (until recently) performance was perfect.

I never really knew why that method seemed to fix performance and i never really questioned it because it worked. I have a feeling its my gpu but Its no more than a feeling. Also now it doesnt seem to be turning back on but in the past sometimes it wouldnt start back up for a few minutes or a day sometimes flipping the switch on the psu would let it boot.

I'm running win7 32 bit
Memory - 4gb ddr2 OCZ Reaper @1066
CPU - Amd Black Ed. quad @ 3.0ghz
GPU - BFG 9800 gtx+
Mobo - Gigabyte half card (forget make)
PSU is a BFG 750