I'd start by lubricating and cleaning the motor. Have you tried running it without the shell? If that doesn't work contact Bachmann, tell them your problem, and they may send you a replacement for free. Bachmann is constantly improving there locos so the new one might have had that problem before the improvements were made since. If you are really attatched to that loco send it to a model railroad repairshop (and no I don't know of any). Good luck!
TGR
