Framerate issues?

I have the PS3 40GB model and it seems for periods of time my framerate will be terrible. I was playing a single player game as the aztecs and about half way through the game everything started going slow and choppy. Moving around the map was tedious and I actually had to hold buttons down until the game would recognize that I pushed them. Battles were normal and it seemed as if some parts of the map were better then others. I saved and restarted my PS3 and it did not help.

Then just as fast as it started, the game ran smooth again. I'll pay more attention next time to try and figure out exactly when it's happening and stopping. Maybe there is something going on..