In my opinion, there's a good probability that a wire has rubbed through the insulation somewhere in or near the drawbar area, and is intermittently shorting out against a frame or some other protuberance. Pop the shell off the tender, and visually investigate all the wiring from the drawbar to the board. If that fails to turn up any damaged insulation, pop the shell off the loco and look for any problems with its wiring.
That's where I would start. I have had several similar problems in the past, and in most cases it ended up being a wire shorting out due to abraded insulation.
