What was found
WHEN AUTOPILOT 'B' SELECTED, THE STABILIZER OUT OF TRIM LIGHT DISPLAYED. AIRCRAFT STARTED DESCENT AND WOULD NOT HOLD ALTITUDE, THE AIRCRAFT CLIMBED SEVERAL TIMES. AFTER DISCONNECTING AUTOPILOT, COULD NOT TRIM FWD USING YOKE SWITCH OR MANUALLY TRIM FWD PAST 4.5 UNITS. FOUND MAIN STABILIZER TRIM ACTUATOR INOP. REPLACED THE ACTUATOR, CHECKS OK.