Anyone playing on a Geforce 9400m?

That's what my integrated gpu is on my macbook pro (mid-2009) and sometimes, playing the game is amazing (lowest possible settings), and others, my fps drops to 5-15, which is unplayable.

Anyone else using this setup and figure out a way to play with no fps issues?

Macbook Pro (mid-2009)
Intel Core Duo 2 2.53ghz
4GB RAM
Nvidia Geforce 9400m
Windows 7 Ultimate 64-bit (bootcamp)