Performance drops as the game progresses...

[Creep Deadson]

I have been playing a while now with no problems up until about the point in the game where you see the conveyer belts. Since that area(or so) the game has been getting really choppy, even in areas where nothing moves. The first time I encountered the spiders, the game ran like a rocket ship, then (with the settings the same), I encountered the spiders before the room with the lockers and the game was running very slow, especially when fighting. Has anyone else noticed this? My computer is 2.0 ghz amd Sempron, 1gb Ram, Geforce 6200 512mb (buiding a new pc as we speek), but this pc plays quake 4 and doom 3 on maxed out quality just not the highest resolution. What gives???