Darko,
I think Mel Effluent might be on to something there
incident in an RN Gazelle many years ago, involving the failure of an internal main gearbox component which allowed the rotor shaft to move vertically, thus changing the pitch on all the blades
I think the governor making you lose RPM is a long shot since you would have most likely heard the RPM decrease, also with less RPM you would have needed to push the pedal a lot more to maintain directional control, and power would have gone way up which would have been noticiable as engine noise.
But hey the only thing I know about a Gazelle is that it has a Fenestron, and that I saw one in a museum in Bournemouth museum years ago.