It works so well, for a little while . . .
 

When I first load the game, it runs very smooth and I have very little lag. After approximately 5 minutes, it's like someone flips a switch and I can barely move my character much less play the game. I then minimize my screen (I use window and not full screen) for 60 seconds and bring it back up, and everything works great again for another five minutes or so. Any idea what's happening and what I can do to fix it?

HP Pavilion dv2700
Windows Vista Home Premium
AMD Turion 64 X 2 TL-62 (2 CPUs) - 2.1 GHz
4 GB Ram
NVIDIA MCP67M - GeForce 7050M