How are you checking your temps and what are the temps when your problem happens? Are all of your drivers all up to date? When was the last time you took a can of air to clean all the dust out of the case?
Sorry I had to ask you would be suprised on how many people don't do that. It is not you PSU this time. Can you check your GPU and make sure your fan on it is going?
And make sure all your over clocking is back to normal. You could just be having unstable hard wear by OC'ing. Let us know what say you. Good luck.