76 C is not particularly hot. You shouldn't see the CPU stuttering until IIRC 105 C. 64 C at idle is on the warm side compared to earlier models.
How hot is the GPU getting?
I guess it could be any number of problems. Hardware fault. Graphics driver bug. OS X problem.
Could it be a bug in this fancy GPU switching which Apple has designed?
Can you replicate the freeze in anything else, eg any other game?
When the game freezes, does the whole computer freeze, or do other apps continue normally?