Having the same problem with 2 of my older 2 rail locos. Same behavior on my track or when running directly from my MRC Control Master 20 Power Center (video). These two engines will run a few inches, then seem to short, then run a bit more. Repeat, Repeat. They run better below 3 volts, very slow. Going above that will ensure this stop/start action.
They are not binding, but in the case of the video below I can see some small sparking and occasionally a small wisp of smoke. Anything I can change? Wondering if they maybe need to be repowered.
Not sure of the make of this one, but the other is a mid-80's Sunset 4-6-2. Since the track can be ruled out, I think it's either the engines (which seems peculiar since it is not just one loco) or possibly the power pack. Could that be?
Any suggestions would be appreciated, except to mount these on the wall forever. They need to run.