Came to this somewhat late, but why not jump in anyway!
IME the 412 ground bounce is usually caused by track/balance problems, and can't always be eliminated no matter how long you try. I've seen it caused by a t/r imbalance. We worked on the m/r track/balance for weeks, with no success in lowering the ground bounce. Then when the t/r balance was done as part of the scheduled maintenance, it was found to be well out of tolerance, and when balanced the ground bounce was gone.
It's a miracle when a 412 has no vibrations anyway, considering that it starts out with the blades in different planes. Sometimes you just have to live with a ground bounce, but I try to get it tracked out. Raising the collective slightly usually helps, as already mentioned.