What was found
IN CRUISE AT FL 340 A AUTOPILOT DISENGAGED. ALL INDICATIONS WERE NORMAL AND FLIGHT DIRECTOR REMAINED IN LNV / VNAV. RE-ENGAGED A AUTOPILOT AND IT OPERATED NORMALLY FOR THE REST OF THE FLIGHT. WE WERE IN SMOOTH AIR WHEN IT DISENGAGED. GROUNDED - YES. ACCOMPLISHED LAND VERIFY TEST REF B737-900 AMM 22-11-00-700-801. ETOPS GROUND VERIFICATION SUCCESSFUL. NO CURRENT FAULTS. NO OTHER HISTORY PASSED 30 DAYS.