Losing torque, and starts skipping, maybe heat related. Damaged gear teeth, in particular if bits floating around, can give intermitant overload, leading to mis-stepping. If the torque is reduced, and there is slipping, then the nylon? gear has come loose on the motor shaft, or some other shaft. Using one of these to wave a needle around in an instrument, is not the same as expecting it to do real work. For your application, what is it's advantage over the very common type of rc servo's?