
The screenshot really doesn't show how bad it looks. When you move around, the grain shifts in a really ugly way.
I tracked this issue down to the r_SSAOQuality setting. When r_SSAOQuality = 3, you get the grain.

With r_SSAOQuality = 2, the grain is gone.
So, the fix is simple. Just add r_SSAOQuality = 2 to the end of your autoexec.cfg file. If you don't have a autoexec.cfg file, use a tool like this to make one.