What was found
AIRCRAFT NOT GROUNDED: PILOT REPORTS AT CRUISE ALTITUDE WITH CMD B AUTOPILOT ENGAGED VTK DISPLAYED ON BOTH ND DISPLAYS RETURNED TO NORMAL ALSO LNAV DISENGAGED HAD TO REESTABLISH THE LNAV ROUTE AND REENGAGED VNAV. THIS OCCURRED FIVE TIMES DURING THE FLIGHT. FMC LIGHT DID NOT ILLUMINATE. MAINTENANCE PERFORMED TEST TO BOTH FMC FLIGHT MANAGEMENT COMPUTER SYSTEM AND OP TEST IAW AMM 34-61-00-710-801. TEST RESULT OK.