What was found
AT CRUISE ALTITUDE, WITH FO'S AUTOPILOT ENGAGED, FMC AMBER LIGHT ILLUMINATED. A/P REVERTED TO CWS AND ALT HOLD. ON FO'S PFD A LANDG ALT AND ON THE FO MFD A VTK ANNUNCIATION WAS OBSERVED FOR ABOUT 30 SECONDS. CAPTAINS SIDE REMAINED NORMAL WITH NO FLAGS. NEVER RECEIVED AN FMC SCRATCHPAD MESSAGE. AFTER 30 SECONDS ALL WARNING FLAGS WENT AWAY AND LNAV/VNAV WAS ABLE TO BE RE-ENGAGED ON FO'S SIDE. GROUNDED - YES. L FMC SHOWS "FMC-R PRES POS LAT RATE" LAST FLIGHT, REF B737-800 FIM 34-61 TASK 801. PERFORMED FIM 34-61 TASK 824, NO FAULTS FOUND. ETOPS GRND VERIFICATION NML.