As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) AFTER TAKEOFF THE PILOT NOTED THAT PROP RPM HAD DROPPED FROM 1900 TO 1790, HE RETURNED TO BASE IMMEDIATELY. AFTER NUMEROUS GROUND RUNS THE OVERSPEED GOVERNOR WAS NOTED TO BE GOING INTO TEST POSITION WHEN NOT COMMANDED TO, THIS WAS INTERMITTENT. THE OVERSPEED GOVERNOR WAS REPLACED WITH A OVERHAULED UNIT, NUMEROUS GROUND RUNS COMPLETED WITH NEW UNIT INSTALLED WITH NO FURTHER OCCURRENCES. ACFT RETURNED TO SERVICE.