As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE PILOT INITIALLY COMPLAINED THAT THE AUTOPILOT WOULD NOT MAINTAIN CONSTANT VERTICAL SPEED. THIS ESCALLATED INTO A FULL NOSE DOWN TRIM RUNAWAY. THE PILOT RECOGNIZED THE PROBLEM AND RECOVERED FROM DIVE. THE PITCH SERVO WAS CHANGED AND THE PROBLEM DID NOT RECUR.