it appears to me like it's losing power. I would suggest checking for continuity between the power pick-up rollers, check the ground pick-ups as well, and the tether. You may get lucky and it's just a loose wire. If you wiggle the tether while it's sitting on powered track, does it cut out?
It's hard to guess why it's doing it, just from the video. The fact that the headlight goes out, means something here. How many 3 rail pickup rollers does it have? Any on the tender?
