One thing I have found out is that the bogie gear casings are very low, and they can get stuck if point motor pins aren't properly trimmed. The low slung Heljan couplings can also get cought on these pins. In a noisy show, the loco might appear to stop with the lights on, the sound of the motor being drowned out by the noise in the show. The loco might then appear OK when taken off and tested.