I read that you don't think its because of overheating, but you should check the temperatures first because i had the same problem(on a PC) and these are generally due to overheating while playing games(high cpu/gpu usage).
So you should download speedfan- http://www.almico.com/speedfan443.exe
and start it before running the game, let speedfan run in background(not inactive in tray but with its window at your desktop), play the game for 3 minutes or so(make sure you exit the game before windows turns off) and then have a look at the temperature graphs of cpu and gpu.
If its overheating there should be steep rise in the graphs towards 100/110 °. If its not anyways post the screenshots of your graphs.