Why is my computer crashing?

Whenever I play games like BF3 or Black Ops 2 my pc will crash. Most times it won't crash but will spaz out with random pyramid shaped colours all over the screen. I suspect it could be my PSU which is a COODmax 500w PSU (really cheap) and my gpu is a Nvidia Geforce GTX 550 ti. My GPU and PSU are only half a year old so I don't really see and wear and tear there.

I'm just wondering if it is 100% sure it is the PSU.

Also I have 8GB of ram. My graphics card runs BF3 at 30+ fps which is fine with me but my card tends to overheat. That is why I think my cheap 500w PSU isn't giving my GPU enough power and pretty much overheats it.