What was found
IN CRUISE FLIGHT AT FL310, MACH .84, THE AUTOPILOT WAS PULSING BACK AND FORTH WITH A CORRESPONDING ACFT PITCHING MOVEMENT. DECIDED TO TRY SERVO B, UPON DISCONNECTING AUTOPILOT, DISCOVERED ACFT WAS OUT-OF-TRIM NOSE UP. COULD NOT TRIM THE ACFT NOSE DOWN WITH EITHER CONTROL WHEEL TRIM SWITCH, CRUISE TRIM, OR MANUAL TRIM WHEELS. THE TRIM WAS INDICATING .5 NOSE UP, WE COULD TRIM AFT (MORE NOSE UP), BUT NOT NOSE DOWN PAST .5 NOSE UP. AFTER LANDING, TRIM WORKED NORMALLY AND WOULD TRIM PAST .5 NOSE UP. REMOVED AND REPLACED AUTOPILOT STABILIZER TRIM SERVO MOTOR IAW AMM 22-11-021-2. ACFT OK TO CONTINUE. PREMATURE REMOVAL/INSTALLATION.