As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED AIRCRAFT PORPOISES 5 DEGREES UP AND DOWN IN CRUISE. AUTOPILOT PITCH COMPUTER REMOVED AND REPLACED WITH LOANER UNIT. THIS PROBLEM HAS OCCURRED IN THE PAST IT IS SUSPECTED THAT THE SERVO DRIVE SPEED IS TOO SLOW. SUGEST ADJUSTING THE SPEED AND PERHAPS MOVE THE MOUNT LOCATION CLOSER TO THE ELEVATOR BELLCRANK TO LESSEN VECHANICAL DELAY IN PITCH FROM COMPUTER TO SERVO.