What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS AUTOPILOT B DISENGAGED WHILE IN LEVEL FLIGHT AT FL380. ALTITUDE WINDOW ON FLIGHT GUIDANCE PANEL INDICATED 50000 AND BOTH FLIGHT DIRECTORS DISAPPEARED. TURNED OFF BOTH FLIGHT DIRECTORS AND THEN RE-ENGAGED BOTH FLIGHT DIRECTORS AND RE-GAINED CAPTAINS FLIGHT DIRECTOR, BUT NOT FOS. ALTITUDE WINDOW SHOWED 23800 AFTER FLIGHT DIRECTORS WERE RESET, ACTUAL ALTITUDE WAS 38000. SELECTED LNAV AND 38000,THEN VNAV AND WAS ABLE TO ENGAGE A/P. MAINTENANCE REPLACED DFCS MODE CONTROL PANEL IAW AMM 22-11-34. INSTALLATION TEST GOOD IAW AMM 22-11-34. OPS TEST GOOD IAW AMM 34-36-00.