99.9% chance it's a tether issue. With the locomotive running slowly wiggle the tether. If it speeds up and the chuffing stops it's a tether issue. Either the tether has a loose wire or one of the pins on the receptacle in the locomotive has a loose pin on one of the tach reader wires. Fix is either to resolder the pins on the receptacle or replace the tether, depending on which one is causing the issue.
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