I actually heard of this too surprisingly. I had a customer that was trying to put mato tracks on our sprockets, it turned out he was using the 58mm shaft gearboxes instead of the 48mm and it was making it jump. The stock HL shafts are 58mm, but asiatam and our chassis require a 48mm shaft with our drive wheels. I HIGHLY doubt this is your issue, but the only other issue I can think of is the drive wheels not being perfectly round. Have you tried watching them from the side instead of head on?