Huey "bounce"
You don't say what troubleshooting was conducted, what was found and the corrective action was taken, nevertheless, here are my initial thoughts:
1. Check all flight controls for integrity, particularly rod-end wear.
2. Check integrity of the fifth mount and beam.
3. Check the transmission mounts for deterioration.
4. Check the transmission dampers both for friction and attachment - they have a habit of pulling the attachment brackes off the pylon structure.
5. Check the friction setting of the synchronized elevator torque shaft (in the tailboom, connecting the L/H and R/H elevators).
Hope this helps, let us know the eventual outcome.