If your system starts overheating, it might be automatically dropping the performance to help with cooling. My roommate's system does the same thing after half an hour or so, so he has to play in front of an open window (and it was -20C here all last week) or it'll eventually just crash on him.

More powerful cards also tend to come with better cooling, but you could get better and cheaper results just upgrading your case fans. It depends on your budget.