I think there is a little nylon puck (for lack of a better term) in the gear that dries out and turns to powder after a while, or the gear strips the teeth. This place has all the parts you could need, but I think //<86TA>\\ used a little nut in place of the nylon pieces. You're going to need to take the motor apart and see what is the problem.
Here is the link to the nylon replacement pieces