Is this solved, or what? Also, how are you sure it's overheating, are you monitoring temperatures? Is the computer shutting off, or is the game just crashing and you exit to windows?
If it is indeed overheating: first thing I would do is to make sure you have adequate air flow through the case, open it up, and clean out any and all dust that has caked on over its course of use. If that doesn't fix it, you can try reseating the heat sink with new thermal compound on the CPU.