Not too sure then.
My initial problem with the 24" was my video card, 7800 GT.
FPS was good, always above 60, but I got mouse lag like you.
Did the DPI thing, got better. Switched to a different DVI cable, even better.
Reformatted, went X1900XTX, flawless. :]
I've read tons of forums, some people have issues with mouse lag, some don't. Don't know what causes it, or how people fix it. Well, in Oblivion, they set render ahead from 3 to 0 and that fix's it for the one game.