Jerky Downhill Performance.

[nachoman]

Downhill jerking is often caused by worm to gear mechanisms where the worm gear can slide back and forth. It doesn't affect uphill running because the load pushes the worm one way. Going downhill the worm gets pulled one way by the wheels and then the motor catches up and it slides the other way.
The solution to the slop is usually a washer or two someplace along the shaft the worm is on. On the old steamer motors where the one shaft did all, it often took a washer at the far end or one at the end of the motor where the worm is, but inside the motor (tricky, not recommended for tyros) or at the bottom end beyond the worm if there was anything for it to push against. If there's a gearbox you might be able to tighten that up.

Usually, this problem was where the motor armature would be forced to one side or the other. Very common in old steamers when the worm was mounted directly on the motor shaft. But, most diesels incorporate a slip joint in the universals, so I doubt this is the problem here. You could have the same problem with thrust in the individual worms on the trucks, but it seems less likely.