Added a solid lower guide to test my suspicion about keeping everything lined up helping with operation. With the guide I was able to easily identify two pads that had drive dogs a bit off center. I replace those and tried a motion test with the drill motor.
Trouble going into reverse from this direction with no guide or rollers on the top, the pads want to shift to one side just enough to try and ride up on the tumbler.
http://youtu.be/rLnZibly4JY
Jim