The problem you guys are having sounds like what was happening to me as well. I run a 5870, and was getting some really bad stuttering in combat, only when WS were triggering.

What fixed it for me was; I was running vsync, but did not have triple buffering enabled (didn't know what it was). After I enabled triple buffering, that fixed the problem for me. Worth a try if you're not running triple buffering.

There is a trade off though; triple buffering fills up VRAM pretty fast I guess, so I have lower frame rates in town now, sometimes dropping to 30 fps but never under. Still 60 in the field and in combat though.