Juank,
I might be out of order here, but you may be getting bouncing because your collective friction is too light and you maybe a litttle bit too gentle with the collective lever. I had a similar case with the Bell 205A-1, the pilot's seat is flexible and if you are holding the collective, as you probably are, any bounce in your seat is transmitted to the collective. You say that your maintenance crew cannot find anything wrong, please ask them if the static friction clamp on the left side of the collective torque tube is correctly set? There is a panel just below the left hand side crew door which gives access to the clamp. I apologise if this has been checked before.