Exactly that. I've never had a problem playing l4d before, and now, i cant play for twenty minutes without my entire system lagging down to at least 1 frame every two seconds. like for example, i type seventy words a minute and its just filling in 'twenty minutes', thats how far behind my type it is. it lasts about fifteen minutes before i can go and play my game again aor watch a movie or even move my mouse and see where it sgoing. i have onno i dea if there are typing mistakes in this post because it's not even a firfth of the way displaying what ive typed. my system has downs sydrome. why??!?!?

I don't know wahat the problem would be , considering ive never had a problem with this game before, and its not even consistent. its like every twenty minutes or so my video card memory decideses to go and get a coffee alongside my ram.

I've got a nforce 8600 gt 2G ram, and intel dual core 2ghz on win xp.

It might be overheating. Try removing the graphics card and removing all the dust that may have accumulated inside. Check the cpu heatsink for dust clogging as well.