What was found
WHEN "A" AUTOPILOT IS CONNECTED THE ACF GOES INTO A STEEP DIVE WITH STAB TRIM "A" LIGHT ON ALL ATTITUDE. WHEN AUTOPILOT "B" IS CONNECTED A/C PORPOISES + - 1,200 FRM WITH STAB TRIM "B" LIGHT ON. THIS HAPPENED ABOVE FL 27.0. IF FLYING AT 27.0 IT MAINTAINED A GOOD LEVEL CRUISE, TX DMI M07-028 PER MEL 22-1 PLACARD INSTALLED. REMOVED AND REPLACED AUTO STAB TRIM UNIT IAW 22-14-01, OPS CHECK OK. REMOVED AND REPLACED NR 3 PYLON VALVE IAW MM 36-11-04, OPS CHECK OK. PERFORMED OPS CHECK OF NR 4 PYLON VALVES AS PER MM 36-00-00, OPS CHECK NORMAL.