I tried out the demo last night. It was a blast. There were some problems with multiplayer, but they patched it later in the night and it is working much better. Great enviroment and gameplay. It reminds me a lot of 28 days later.
Source games are CPU based and sometimes an otherwise stable (as in Orthos, Prime95 etc) overclock can cause framerate issues. I discovered this this week. In TF2, L4D, and etc., I was experienced low framerates that disappeared when I dialed back my overclock (E6400) from 3.2 to 2.8 ghz. That may be your problem.
I can't wait to play some more L4D tonight.