Well, it could be the power supply. Download HWmonitor and check if the volts/watts going to your components are up to par. According to newegg your PSU doesn't appear to be 80%+ certified, meaning though it says it's 650W it won't always (or ever, rather) deliver that much power to your rig.
But have you stress tested the GPU and CPU? 6 or so hours for either? Generally speaking the GPU crapping out in the midst of gaming will show signs of artifacts, but not always, like when it's not getting enough power.
You should try to eliminate all of the other possible culprits one step at a time before you find the little sneaky bastard that's been causing your problems. But my guess would be that it may be the cheapo PSU