L4D2 Lag Problems

Ok, so I'm wondering if anyone else has this problem, or has seen a solution.

When I start a map, the game is graphically on/off laggy, meaning the server is fine but I'll go from 15 fps to 300 fps five times a second. It's unplayable basically. However, when I change the video settings (say to 0AA to 2x AA) the lag goes away and I do 200+ FPS just fine and dandy until the start of the next stage, then the exact problem comes back.

What I've been doing is right at the start and each stage is go into Options-> Video-> Advanced -> AA Settings and cycle between 0 and 2x AA. I choose to edit that setting because it's one that makes the lag stop, and it's the fastest and the difference between the two doesn't bother me.

SO, has anyone heard of an issue like this? I can't think of what it could be, and it's happened since the demo, I just haven't been bothered enough until now to find a solution.

Note that I'm running Win 7 RC 7100 64bit, with the latest ATI drivers, game fully updated.