What was found
LEFT FMC FAILED. ACFT GROUNDED. WHILE ON DEFERRAL DURING CRUISE FMC ALERT, MSG VNAV DISENGAGED, FMA SHOWED FMC SPEED/LNAV/ALT HOLD. SPEED MODE ILLUMINATED ON MCP. VNAV WOULD NOT REENGAGE UNTIL WE BEGAN TO DESCENT, THEN ALL VNAV FUNCTIONS RETURNED TO NORMAL. REPLACED LT & RT FMC COMPUTERS. OPS CK GOOD. REF AMM 34-61-00 & IPC 34-61-02B, 34-61-02-11, 34-61-02-09, 34-61-02-04, & 34-61-02-99.