MTH 2-8-0 Jerky motion at slow speeds

Repainted? I'd be pulling the shell and looking at the tach tape on the flywheel and the optical sensor.

Just my $0.02

Yes repainted but only the cab and tender, and I had pulled the shells from the locomotive to do it.  The actual locomotive never went near paint.

Check the gearbox.  Remove the cover, clean out the old grease, inspect the area for cracks or foreign matter, then add new grease.

You could also remove the motor and see if the wheels turn without binding, if they still bind it could be the siderod linkage.

Check the distance between the tach reader and the flywheel.  Someone said to use a dime, if it won't fit it's too close, if it's too sloppy it's too far away (or you could actually measure the gap).

Thanks Bob, I will check these things tomorrow.

Whilst you are checking things, make sure that the 5 mm bolts that hold the rods to the drivers are snug. Do not overtighten. Also, make sure the eccentrics are properly positioned.

If you can get at the motor, turn it by hand until it binds, then stop turning. While it is bound, wiggle each rod independently. Most of them will have some play. The one that is tight will lead you to the problem.