Sudden drop in gaming performance (FPS) on a mid-2010 iMac
I had this and fixed it!

Basically something was overheating.  It would freeze as it hit the overheat temperature and then resolve once the temperature dropped about 5 degrees celsius.

So I downloaded some random fan speed app (link below), cranked it up to full speed and haven't had a problem since!

http://www.eidac.de/