This is what happens to me once or twice a day, depends how much I play..
So I'm playing TF2 normally.. then the game starts to 'stutter'.. like there's massive lag and the fps drop to like 1/2 fps, when my character moves a bit, it 'skips' back to the old place, those are fast +- 0.5 sec skips, like repeating the picture over and over again, then it goes laggy again, then again the skips, then it gets better, but still laggy, about 5 fps I'd say, those are in totally random order.
What I do to try and fix it, is push the windows button on the keyboard, so the screen goes back to windows, and then I go back to TF2 by clicking the button.. I gotta wait some time when it pops up tho.. that's a problem also, whenever I minimize TF2, it needs about 10 seconds before it maximizes to fullscreen, I hate that because I like to search for some things or play another musiclist when the round is done and I have some time.
I've disables automatic updating of my firewall, that helped because I had the lag/freezing more a while ago.
CTRL-ALT-Del helps also I think, but it probably has the same effect as Min/Maximizing.