I've maintained my Bachmann Big Hauler locomotoive (a 4-6-0) but within the last 8-10 days, the motor has started producing occasional "whirring" and the problem seems to be getting worse.  The whirring when it occurs (which isn't always) seems to be accompanied by inconsistent operation, i.e., the train speeds up and slows down with about a 1 second oscillation period.  I've tried cleaning track and wheels, assuming it's an electrical contact problem, but the problem continues to be intermittent, e.g., every other time I operate the train.  The only other problem that comes to mind for me is that it's a motor problem.  Can you offer any advice?