As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT, OBSERVED "VTK" ON CA'S MAP. DISPLAY ACCOMPANIED BY NO FMC. RAN QRH, NORMAL OPS RESTORED. PROBLEM OCCURRED AGAIN MORE THAN AN HOUR LATER. RAN QRH AND LEFT FMC SWITCH IN "BOTH ON R" FOR REMAINDER OF FLIGHT. GROUNDED - YES. PLACED FMC SWITCH BACK TO NORMAL AND PERFORMED AN OPS-CHECK OF THE LEFT FMC PER AMM 34-61-00-710-801.