One more thing to check: It could be that your CPU is overheating. Try downloading CoreTemp at http://www.alcpu.com/CoreTemp/ and then running a game in windowed mode (or using something that stresses the CPU) to see how hot your CPU gets. If it goes over 55-60C, then that's probably the culprit.
You may also want to do the same thing with your GPU by running FurMark for a bit and seeing if your GPU temps go above 80C or so. Your GPU may have a defective cooling unit.
If it's the CPU, then try reseating the heatsink and fan. You may also want to try reapplying thermal paste to your heatsink. If it's the GPU, then it'll probably need to be RMA'd.
But yeah, it may also be the power supply. Do you know its brand name and/or model number? I'm assuming from the odd imbalance of parts (weak graphics card + overkill CPU) that you bought a prebuilt, so the PSU probably sucks. If it is a bad PSU from a generic or third-rate brand, then I wouldn't even bother RMAing it; just grab a SeaSonic or something to replace it.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."
-Sean Malstrom
Essay Pro