I'd bet money it's an addon or combination of addons. Last time it happened to me was when I had a hacked up copy of Scrolling Combat Text showing damage from my alts and forgot to set the animation mode to "FPS-independent." If you haven't tried yet, give disabling your addons a shot and see if it helps. Yeah, it'll suck for a while but you can test it on target dummies as that's technically combat and it's safe. :P