What was found
AIRCRAFT WAS GROUNDED: AIR INTERRUPTED BACK TO DFW ACCOUNT OF MAINTENANCE, VERTICAL NAVIGATION ISSUE. INTERMITTENT SPEED LIMIT FLAG ON PRIMARY FLIGHT DISPLAY WHEN IN VIEW VERTICAL MODE CHANGES ON FLIGHT MODE ANNUNCIATOR. VERTICAL GUIDANCE ON FLIGHT DISPLAY DISAPPEARS. AUTOPILOT IF ENGAGED REVERTS TO CONTROL WHEEL STEERING P. FLIGHT DID NOT DECLARE EMERGENCY AND DID NOT LAND OVERWEIGHT. FOUND MAINTENANCE MESSAGE 27-31004. REMOVED AND REPLACED LEFT ANGLE OF ATTACK VANE PER AMM 34-21-05.