What was found
CLIMBING FROM 350 TO 370 IN VNAV WE RECEIVED A VNAV INVALID - PERF MESSAGE IN THE SCRATCH PAD AS WELL AS A VTK ON INBD DU'S VNAV DISCONNECTED AND WE RE-INPUT COST INDEX AND WE WERE ABLE TO REENGAGE VNAV. GROUNDED - YES. PERFORMED VNAV TEST REF B737-800 FIM 34-61 TASK 845 STEPS C (1) - (3). ALL REQUIRED TESTS PASSED W/ NO FAULTS NOTED AT THIS TIME. FMC SOFTWARE INFORMATION IS CURRENT IAW B737-800 AIPC 34-61-02. OKAY TO CONTINUE.