What was found
AT FL300 APPROXIMATELY 45 MINUTES INTO FLIGHT, AUTOMATION REVERTED TO 'CWS R' FOLLOWED SHORTLY BY A 'VTK' MESSAGE ON CAPTAINS ND. BOTH MCDU'S DEFAULTED TO MENU, NEITHER ONE DISPLAYING 'FMC' OPTION AT 1L. WE SELECTED HDG AND ALT HOLD, BUT BEFORE WE COULD TAKE ANY FURTHER ACTION, BOTH FMC'S RESET. WE NEVER LOST EITHER ND. GROUNDED - YES. LEFT FMC HAD 'PRES POS LAT RATE' FAULT LAST LEG, NO OTHER OCCURRENCES OR FAULTS. NO ADIRS FAULTS LOGGED. ACCOMP B737-900 FIM 34-61 TASHS 824 & 836, 34-21 TASK 801, B737-900 AMM 34-21-00 AND LAND VERIFY. ALL CKD GOOD. GROUND VERIFICATION SUCCESSFUL.