I think this is a combination of a couple problems, the first being what David touched upon. The ZWL is a zero start transformer. So as David replied, your engine is beginning to move before there’s enough current to fully energize the E unit’s coil. So David is 100% correct on this. Second, possibly dust & debris on the track. To simply verify this issue, get the locomotive running in one direction or the other, and flip the lever to turn off the E unit. Observe its operation to see if you experience stalls that also make the headlight blink, or go out completely. This would indicate a loss of connection. Then suspect sliding shoes, dirty wheels, dirty/dusty track, etc. …..