As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE AIRCRAFT WOULD PORPOISE +-100 FEET DURING CLIMBOUT WITH THE AUTOPILOT ON USING THE THUMB WHEEL. THE AIRCRAFT WOULD NOT HOLD ALTITUDE IN LEVEL FLIGHT WITH PITCH HOLD SELECTED. CREW REFUSED TO HAND FLY THE AIRCRAFT AND THE FLIGHT RETURNED TO AN UNEVENTFUL LANDING AT MEM. MAINTENANCE REPLACED THE PITCH AXIS COMPUTER AND THE AUTOPILOT CONTROL PANEL. OPERATIONAL CHECK OK. AIRCRAFT RETURNED TO SERVICE.