What was found
AT CRUISE ALTITUDE OF 34000 FT, .78 MACH, AT APPROXIMATELY 0112Z, VTK FLAG ON CAPTAINS SIDE FOLLOWED BY DUEL MCDU BLANKING AND LOSS OF LT NAV TRACK. F/O SIDE WAS MASTER, VTK FLAG THEN RESOLVED ITSELF WITH NO PILOT ACTION AND MCDU'S RECOVERED. FMC FLIGHT PLAN RETURNED. SUBSEQUENTLY CAPT USED FMC TO ATTEMPT A "DIRECT TO" AND THE ENTIRE ANOMALLY REPEATED ITSELF THEN RECOVERED. AFTER RECOVERY, DID NOT USE CAPTAIN SIDE MCDU KEYBOARD AND ALL WAS NORMAL. LEFT IRS SHOWED 2 MILES OFF. ACFT GROUNDED. FOUND MULIPLE FAULTS FOR FMC CHANNEL A. REPLACED NR 1 FMC. ALL OPS NORMAL. REF MM 34-61-02. FMC SYSTEM TEST PASS, REF MM 34-61-00-730-801.