What was found
ON TWO OCCASIONS HAD "VTK" FLAG, LOSS OF VNAV & LNAV, AND BOTH MCDU SCREENS BLANK. BOTH TIMES BOTH FMC RESET THEMSELVES AFTER 30-60 SECONDS WITH ALL FMC DATA RESTORED. ON ONE INSTANCE LEGS PAGE COULD NOT BE MODIFIED UNTIL SWITCHING TO RTE NR 2. GROUNDED - YES. USING OBSERVED FAULT FIM FOR VTK FLAGS, PERFORMED FIM 43-61 TASK 801. NO DEFECTS NOTED. BOTH FLIGHT MANAGEMENT COMPUTERS OPS CHECK GOOD IAW AMM 34-61-00.