no, it's not that.
With configuration 800x600, Texture compression = "ON" and texture resolution = "Medium", i have about 5 seconds of good working mouse and after that it goes 'bout 0,7 FPS.
Theres one more strange thing about the mouse: when it slows down, the cursor doesn't want to go to right side of the screen - for example in OPTIONS(Video) menu I can't point on arrows to change anything 'cause cursor stops about 100 pixels from right side of the screen.
Only a guess: isn't it something with videocard memory?
It reminds me problems with some card and some 3d Windows games.
Maybe i'll try on GF3 TI200 but i don't know if i'll have a time for that today.
sorry for my english - i'm polish and just got up from bed