What was found
AUTOPILOT DISCONNECTED UNCOMMANDED DURING DESCENT. AIRCRAFT EXHIBITED UNCOMMANDED PITCH TRIM INPUT RESULTING IN NOSE-DOWN ATTITUDE REQUIRING SIGNIFICANT MANUAL CONTROL INPUT. AFTER PULLING PITCH TRIM CB AND RESETTING AUTOPILOT, AIRCRAFT THEN EXHIBITED UNCOMMANDED NOSE-UP ATTITUDE ON AUTOPILOT RE-ENGAGEMENT. ISSUE SUSPECTED WITH PITCH TRIM AND/OR AUTOPILOT SYSTEM. THIS ALSO HAPPENED WITH THE AUTO PILOT DISCONNECTED, AND PITCH-TRIM CB PUSHED BACK IN. THE PILOTS TRIED TO MANUAL TRIM THE AIRCRAFT WITH THE MANUAL TRIM WHEEL, AND PITCH TRIM CB PUSHED IN. AT FIRST COULD NOT TRIM THE AIRCRAFT AND THEN A SUDDEN ABRUPT PITCH UP FOLLOWED BY AN AGGRESSIVE PITCH DOWN. ELEVATOR SERVO WAS FOUND TO BE THE ISSUE WITH THE SYSTEM AND REPLACED WITH OVERHAULED.