Did you check the stuff I mentioned previously? A loose motor mount or a gear mesh issue can cause what you see.
A close physical inspection of the rods and wheels, specifically the traction tires, will be high on my list. When you get to a tight spot in the rotation, check all the drive rods for looseness. If there is one that is tight, see if you can determine why? Another thing, as you rotate the wheels, see if any axles are bent. The wheels should track straight and true, no wobble as you look as they rotate. If someone as dropped it, you could have a bent axle.
