As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AT FL350 WITH A/P ENGAGED AND ELEVATOR IN A OR B MODE, A/P PITCHES UP AND WILL NOT HOLD ALTITUDE. (BEGINS ALTITUDE OSCILLATION AND OVER CORRECTS WITH TRIM). AUTOPILOT DEFERRED PER DHL MEL 22-1 CAT B, DMI 6023 PLACARD INSTALLED. COULD NOT DUPLICATE ON GROUND. SWAPPED CAPTS AND AUCX GYRO FOR TROUBLESHOOTING. OPS CHECKS GOOD IAW B727 MM 23-11-01 AND 34-22-21. DMI 6023 CLEARED.