Had this issue the other day. A few things I tried to fix it.
1. Disable steam cloud for L4D2. Didn't work for me but others say it did.
2. Verify integrity of game cache
3. Go to your steam directory and delete the file called ClientRegistry.blob and restart steam.
4.(This one worked for me) While in game, open console and type cl_updaterate 50 Sometimes it's set at 100. Turning it down to 50 didn't seem to have any gameplay changes with me. Seemed smooth still. So give these things a try and let me know if they work.