Setting the Vsync to Count to Every V Blank fixed the rotation issue with the mouse.
I am very curious why the mouse rotation speed is affected by the Vsync... it was very good that you figured this out so quickly.
I guess any future configuration for mouse rotation might have to take that into account.
I know for FPS we calculate amount of change over the frame's duration (Delta Time between each frame) to do certain motions, and various calculations in game engine design... but the Vsync thing sort of baffles me. hehe
Thanks again!