Could be: oil in the magnetos.
The "governor" (it doesn't govern, it is easily over-ridden) has a sensor in the magnetos which can be dirtied by oil. Had this happen straight after a 100H check whilst doing the ground run - the engine/rotor started to run away, so closed throttle. Shut down. Restarted with engineers watching avidly and it happened again.
Turned out that an engineer had been too liberal with oil in the magnetos.
If it ain't broke.........
Check and let us know.