What was found
DURING CLIMB OUT, THROUGH 17000 WITH ALL PARAMETERS STABLE, AUTOPILOT B CONNECTED, ALL FMC ENTRIES CORRECTLY ENTERED AND STABLE FOR SEVERAL MINUTES. WITH NO MANUAL INPUT TO EITHER FMC, AIRCRAFT EXITED VNAV AND ALT HLD ILLUMINATED. THRUST LEVERS CAME BACK TO IDLE AND AIRCRAFT BEGAN A SMOOTH TRANSITION INTO DESCENT. INTERVENTION AT THIS POINT CWS. DOUBLE CHECKED ALL PARAMETERS, REENGAGED AUTOMATION WITH NO FURTHER ISSUES. JUST TO CONFIRM THERE WERE NO FMC OR AUTOMATION MANUAL INPUTS WHEN THIS ANOMALY OCCURRED. ACFT GROUNDED. ACCOMPLISHED BOTH FMCS OPERATIONAL TEST AND OPS CHECKED OK IAW AMM 34-61-00-710-801. NO FAULTS NOTED, ACFT OK FOR SERVICE.