Computer freezes while playing WoW

First off, my computer has been overheating since the day i bought it, it would get so hot that the computer would shutdown. But i fixed that issue (or so thought) by opening up my case and placing a small fan inside. I have probably been playing wow on this computer for about a year now with no issues. Recently, after about 5 minutes of playing wow the game screen will freeze, I can move the mouse around still, but the keyboard / keyboard commands are un-responsive. And If i keep the computer on for a few minutes after it freezes it will eventually go to a black screen, and I'm forced to manually restart or shutdown. I'm about 75% sure its overheating issues. And if it is, should i get a new power supply? Or try to get a better cooling system? Not sure if this helps or not, but if I'm searching the web or listening to music or w.e, the computer could probably stay on for at least a half hour before freezing. And when i play wow, it usually freezes about 1-2 minutes after i log into WoW. Thanks in advance for any advice.
 
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