What was found
AT FL 390 IN BOSTON CENTER RVSM AIRSPACE AT .76 MACH THE AUTO-PILOT LOST 500 FEET OF ALTITUDE AFTER THE AIRCRAFT PORPOISED 60 - 80 FEET. THE AUTO-PILOT WAS THEN DISENGAGED AND THE AIRCRAFT WAS BROUGHT BACK TO FL 390. THE AUTO-PILOT WOULD NOT ENGAGE AND THE AIRCRAFT REQUIRED MORE THAN NORMAL FORWARD PRESSURE ON THE CONTROL YOKE TO HOLD THE ALTITUDE AT 390. THE ELECTRIC TRIM AND TRIM WHEEL WERE REPORTED AS LOCKING UP. AFTER DESCENDING TO FL 230 THE TRIM AND AUTO-PILOT REVERTED BACK TO NORMAL OPERATIONS. THE CREW DIVERTED THE AIRCRAFT TO KALB WHERE A NORMAL LANDING WAS MADE. MAINTENANCE REPLACED THE ELEVATOR TRIM SERVO MOTOR. SYSTEM OPERATIONAL CHECKS WERE NORMAL. THE AIRCRAFT WAS RETURNED TO SERVICE.