what is causing this stutter problem?

[Tonglebeqk]

http://www.youtube.com/watch?v=ciYCwkQOwr4

At around the 30 second mark, the CS:Source video test stutters briefly. It does this _every_ time in the exact same spot. It's not a big deal, but I do want to know what could cause something like this. The CPU scheduler is tuned, I've tried using the BFS scheduler as well. The I/O scheduler...have tried them all, but for some reason it always stutters at the exact same spot. Later on in the video test, when it loads up fire graphics, it stutters as well (I didn't record this part though). Again, the same spot.

How can I figure out what's causing this? AMD64 on gentoo, 190.42 drivers, gtx 260 w/896mb. direct rendering: yes, blah blah blah. Even overclocking the card did not help at all, so perhaps this has nothing at all to do with nvidia. But I just want to know what it might be, and hopefully if I figure this out, Bioshock will be a lot less laggy.