Camera Snapping When Mouselook Starts

Anyone know how to fix a problem where when I right-click and start using mouselook, the camera suddenly jumps a rotation proportional to the distance the mouse is from the center of the screen then drives normally?

For example, if my mouse is halfway between the center of the screen and the right edge, when I hold right-mouse and move slightly, my character will suddenly rotate 90 degrees to the right.

Likewise if my mouse is halfway between the center of the screen and the top edge, and I hold right-click then move slightly, my camera will pivot up 45 degrees.