The passenger window starts down on 2006 XK8 when lowering top but kind of stutters and stops before going all the way down. Driver window doesn't move. North windows work normally when raising/lowering in normal use?

 

meant BOTH windows work normally otherwise!

 

A. Try to reset the windows by lowering them all the way down and continue to press down until you hear a "click".

B. Weak battery. Even if it starts and runs fine, a weak battery can cause all sorts of gremlins.