As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) PROPELLER NOT REGULATING OIL PRESSURE IN FLIGHT. RPM WOULD SWING WITH POWER LEVER CHANGES. UPON INVESTIGATION IT WAS DISCOVERED THAT AN O-RING AND BACK-UP RING COMBINATION INSTALLED ON AN INTERNAL TRANSFER TUBE ASSY IN THE PROPELLER HUB WERE REVERSED CAUSING THE O-RING TO CHATTER, A SUBSEQUENTLY WEAR WITH HYDRAULIC PRESSURE FLUCTUATIONS AND THEN LEAK CAUSING A DROP IN HYDRAULIC GOVERNING PRESSURE.