Brightness/Contrast "Flickering," for lack of a better word

Basically, my brightness seems to twitch up and down in certain areas. Almost seems like HDR is trying to adjust, but not doing it smoothly. In my other Steam games (HL2, L4D2, Portal, etc), I’ve never seen this issue, but it’s been bugging me for a while now in Gmod, even after a full cleaning before the recent update.

Here’s a video I uploaded to Xfire- low quality, but definitely demonstrates the problem-

http://video.xfire.com/450372-4.jpg

Exactly the same problem I’m having, as seen here: