My advice is:
1. read the procedure for this malfunction in the RFM, and
2. do what it says.
Then you can't go wrong. Problems arise when pilots think they know better and do something else.
With redundant digital engine controls in modern helicopters being so reliable, the chances of this malfunction happening are virtually zero. Whether you have a digital engine control or hydro-mechanical, the initial action is always the same. Keep flying. Collective follows the NR. If the NR is going high/low, follow it with collective. There is no real urgency to deal with this malfunction if the NR is maintained within limits.