I have a theory I've never been able to prove but seems possible. If the slipper is slipping too much it creates a lot of heat. That heat gets transfered to the top shaft via the slipper plates. Since the top shaft is also the gear at the top of the tranny that gear is getting warm too which in turn softens the plastic idle/middle gear in the transmission. In simple terms, make sure your slipper isn't too loose. Just another thing to check along with the other great suggestions above.