Many of the 412's I have gound run have been prone to some form of bouncing. It has always happened on concrete and starts at about 95%+ with min pitch. Invariably the aircraft has been lightly loaded (empty actually). Certainly there was the thought that this could develop into full ground reasonance if it were allowed to continue Raising the collective slightly seems to be the cure. It was very uncomfortable and I am not able to say whether the condition is divergent, or not.
I have never experienced this problem on either a B212 or a B205 although I have experienced collective bounce on a B205. The situation there was a gentle run on on a bank of shells/stones caused by insufficient friction on the collective.
Hope this helps.
NC43