A while back I had a very similar bug with CS:S using a Nvidia card. It turned out to be a bug with the control panel. If I used the card driver
to manually set settings in the game, it ran terrible and skipped around. I fixed the problem by going into the nvidia control panel, selecting CS:S and setting it to all default settings, from there the game ran perfectly, and I set my graphics options from within the game.
I can't test if this is still the case, I haven't used Nvidia for over a year, but the card was very similar.
This may not be the case but I think it's worth posting.