After shimming the motor with some small, thin washers I had laying around, I'm happy to report that the lead A-unit is now running buttery smooth, even at speed step 1 (out of 100)! The issue apparently was that the worm was rubbing against the bottom of the gear box. Thanks to all for helping me get this far!
I think the worm gear in one of the trucks on the trailing A-unit has a crack in it - see attached image. Even after shimming the motor with the same type of washers, binding still occurs. The interval between binds is also quite a bit longer than it was for the motor/truck on the lead A-unit. This leads me to think it may in fact be the cracked worm gear that's the issue for this one. Out of curiosity, I checked a truck on the B-unit to see if they had the worm gear, but they do not.
Any idea where I could get a new worm gear for this set? And how hard is it to pull apart the truck and replace that gear?
