what car do u run? IT is possible if you dont have a square motor moutn that you are hitting your spur/pinion gear on a slight angle? That woudl put added pressure on the armature and wear on the pushing... But you would think you would see this wear show up on the spur or pinion gear.
Maybe post a detail picuture of your chassis. Hi res of you motor mount,motor and gear mesh. Maybe it would be easier to see your problem with a photo