Well since you changed everything to the other chassi and it still did it… i would think it has nothing to do with the chassis. I would take a look as someone suggested that one of the drive gears? has to be the culprit. The part that does not make sense is that it takes one and a 1/2 rotations to see the problem that goes agains it being the gear which would complete a revolution in one turn?? could it be a problem with the crank or axle?