It could be a problem with the memory used.
If you are using a 32-bit OS you are generally limited to 2GB of memory space. If your game needs more than that it will use the swapfile, which slows things down.
I'm using Vista 32-bit. 2 GB RAM.
It could be that those parts of the campaign are more graphically intense, and this is not always noticable to the player. Do you have a kick ass machine? Does it help to turn down the graphics?
Sager IF90, 8600 mGT, 2GB RAM, dual core 2Ghz. Not kick ass...anymore. It does help when I turn the graphics down, although I'd like to find out if I dont have to.
Oops I should have read your post better. You said its only L4D, so the other games you're not having an issue with, are they graphically intense aka taxing your GPU much?
TW: Medieval 2, Supreme Commander, and Demigod are probably the most taxing games, but they play consistant (well or poorly)