As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR TURN BACK, DUE TO AUTO PILOT WILL NOT STAY ENGAGED WITH ELEVATOR SERVO ON. THE ACFT PITCHES UP AND DOWN AND DISENGAGED IN VERTICAL SPEED OR ALT HOLD MODES. MX TROUBLESHOT AND FOUND WEAK SHIELD GROUND AT GROUND STUD 1385, REPAIRED GROUND SHIELD AT STUD 1385 IAW SWPM 21-21-1. REPLACED AUTO PILOT CONTROLLER, OPS CHECK GOOD IAW MM 22-10-2. ACFT WAS RETURNED TO SERVICE.