5870 + slow FPS in source engine

[fishingcat]

My 5870 is getting strangely low fps in CS: Source and Portal 2. Benchmarks show I should be getting well over 100 fps, but when I ran the source benchmark I averaged 55.7 fps. This wouldn't be too much of a problem, but I regularly see my FPS drop below 30, causing noticeable slowdown. My card seems to be fine in other games though, getting decent fps in arma II and crysis 2 at 1920x1080 maxed out.

 

[Necc]

Try restarting steam, reinstall drivers. my 5870 did the same in l4d2 some timeago.