So I'm playing "Stalker SoC" on my e5200, 9600gt with 2 gigabytes of ram and the game is set to max with full AA. It was doing between 25-75 fps (normally around 40 or 45) then, when I'm underneath the bridge in Cordon where the soldiers irritate you for money (yes, I killed them) it drops suddenly to 15 fps. It clears up afterwards but when I started the game the next day it does it agian. After a while it goes down to 2 fps when I get to the Garbage. It couldn't be a problem with the graphics card because it plays fine on "Crysis" and "Rise of the Argonauts". I don't know how to get rid of it. I've re-installed the game 3 times already. Has this happened to any of you before and does anyone know how to fix it??
Note: I had already finished half the game with smooth frames per a second before this happened.
Thanks in advance.