What was found
AN HOUR INTO FLIGHT AP "B" DISCONNECTED. REENGAGED HELD FOR 15 MINS THEN FAILED. SWITCHED TO AP "A" WHICH WORKED. AN HOUR LATER TRIED AP "B" AGAIN IMMEDIATELY FAILED. TRIED AGAIN-FAILED. FLEW REST OF FLIGHT ON AP "A". GROUNDED - YES. PERFORMED DIGITAL FLIGHT CONTROL SYSTEM (DFCS) BITE PROCEDURE, B737-9 AMM TASK 22-11-00-740-814 AS WELL AS AMM TASK 22-11-00-730-801, IN-FLIGHT AUTOPILOT DISCONNECT FAULT REVIEW. NO FAULTS FOUND AND THE A/P B SYSTEM CHECKS OK. PERFORMED AMM TASK 22-11-00-700-801 LAND VERIFY TEST NO FAULTS FOUND FOR B AUTOPILOT.