It's a curious one that. The only thing that might cause it, in my opinion, is the way the motor coach chassis is constructed; as the drive is only on one bogie with a very large lump of diecast behind it, and no motored trailing bogie, the inertia of the weight, plus the flywheel might cause the motor bogie to slip slightly followed by the unit regaining traction, then slipping, then getting traction and so on. I think the motor coach would have avoided this by having two powered bogies and the weight situated in the middle of the coach instead of up front, thus equalising the above forces by providing more traction and grip. However, the model would suffer even more from the daylight in the passenger compartment issue with a chunk of metal blocking the daylight!