- Check the pinion's mesh with the main gear
- then check the main gear...give it a feel and make sure it's not out of round or shorn anywhere

If even a *single tooth is shaved off, it'll skip per revolution causing the bouncing you're talking about.

- Additionally make sure you've done the sticky collective fix to rule that out as a possibility
- Out of track blades and slop in your links will also cause the blades to flip back in and out of track simulating a sticky collective problem. You'll hear sort of a "whoomp" the blades will snap into track and the rig will shoot up like a rocket