I would guess that the gear mesh is too tight or that the ring gear is dirty or damaged.
I'm wondering if the problem would go away if you loosen the mesh by one complete turn of the little "mesh adjustment screws. If the problem goes away then you need to set the mesh again.
Hint: If you loosened the mesh screws by one full turn, tighten the mesh by up by 7/8 of a turn and check for the issue again
But then again, I had that happen to me and it ended up being that an ant had gotten in somehow and stalled the mount.