Ed,
I have had good luck talking with the folks at TrainLi. You may be able to order a part after talking to them rather than worrying about shipping your whole loco. Their website has some troubleshooting tips, too, that have saved me a good bit of money putting a 30+ year old fleet back into operation. As an aside, have you opened it up to see if there is something obvious, like a piece of debris, rattling around in their?
Aloha,
Eric