My 2 cylinder Mich Cal did that once and I found that one truck had distorted itself into a parallelogram instead of a perfect rectangle. A couple of bolts had loosened. That caused a gear to jump teeth. I aligned the truck with perfectly square corners and made sure the bolts in both trucks were snug, lubricated the gears and shaft journals and the problem went away. It's hard to diagnose your problem with out seeing or hearing it. But, perhaps my idea will help. Good luck.