1. Not really likely, but you could have tested with 2 units providing faulty power to the PC.
2. Heat maybe not, but the driver crashing and recovering causing a freeze is totally possible. It's been happening to me with a stock 580 in GTA4. I just ignore it.
3. I just had a friend come to me about random lag spikes in Skyrim and simply changing his forced Vsync setting fixed it. In general, any game can cause problems as every rig is different.
4. If you haven't physically disabled system restore, volume shadow copy, and defrag any one of these can be started (or all at the same time) causing reads and writes to the drive to be horribly slow.
There are also any amount of different problems in the software causing whatever it is that is happening in your situation. You could have 2 identical rigs and one still have problems. PCs can just be that way. When that happens it just takes patience (which is hard and frustrating) to figure it out.
For instance, I'm having a serious problem with Windows 8 and periodicly the start menu freezes on me. I can't click anything, can't shut down my PC or anything. I have no idea what is causing the problem but it's there and if I cared enough I would sit down to figure it out. My best guess already is trying to force a higher amount of supported RAM on the board. Thing is, 7 never did that.