So I've start the game and play for a couple of minutes and have a stable fps of 45-50+ Then suddenly it drops to 19-22 fps and stays there until I either go to the menu and wait a few seconds or if i change a setting. I've tried to lower the graphical settings and resolution but either way it gets this fps drop after a few minutes.
any clues how I could fix this?

Specs:
Win XP 32-bit
Intel Core2 Duo CPU T8300 @ 2.40GHz
(2x2 GB) ~3,18 GB RAM
2x NVIDIA GeForce 8800M GTX 512 MB