Well the framerate seems to be fairly stable on 60fps (tied to screen refresh rate) occasionally dipping to 40. When there is a glitch the FPS indicator flicks yellow, but the number doesn't change for long enough for me to read it.

I've turned off the windows indexing service just in case.

I've rebuilt the audio cache with the console command snd_rebuildaudiocache

I can't see any other process using the CPU or disk queue during the game, although I do note that L4D2 does max out the CPU for periods.

I verified the integrity of the local file cache, and it found 3 that failed and were re-acquired. Didn't fix the issue though.

Just running around in circle with not much happening is enough to cause the glitching. Weird.