It sounds like it is an open circuit or an intermitant connection. Check the wiring circuit for continuity.
What I found?
Mine was the low rpm controller located under the left side of the instrument panel.when open it is attached to the panel that is swung open it’s a small grey box.
first the hr meter stopped working on a flight. I decided to fly a known time and the low rpm warning came on during the low rpm test when the collective was raised while at 100%. We took a used controller and hooked it up and the problems were resolved.