As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THIS PITCH COMPUTER WAS A FACTORY REPAIR WHICH UPON RETURN FAILED IN THE SAME MANNER. WHEN INSTALLED THE AIRCRAFT WOULD PITCH UP AND DOWN 5 DEGREES WHILE THE AUTOPILOT WAS ENGAGED FOR CRUISE. THIS AUTOPILOT SYSTEM SEEMS TO BE PARTICULARLY SENSITIVE TO ANY AFT LOADS WITHIN THE ENVELOPE AND COULD BE A FACTOR IN THE PORPOISING PROBLEM. THE UNIT WAS REMOVED AND REPLACED WITH A LOANER UNIT.