Go into your counterstrike:cstrike folder and clear out the folder called downloads and only that one.

for some bizzare reason css doesnt automatically clear it out and after it gets to about a 1000 temporrary files in it, it starts to lag your game. I just cleared out 3,000 temp files from it and my game is now running smoothly again

might be a good idea to vertify your game cache and defrag as well after clearing it out