What was found
DURING 3 HRS AT CRUISE ALTITUDE, THE LEFT FMC FAILED TWICE, INTERMITTENTLY AND FAILED THE THIRD TIME UNRECOVERABLY. FLIGHT COMPLETED VIA QRH PROCEDURE WITH FMC TRANSFER SWITCH SELECTED BOTH ON RIGHT. ACFT GROUNDED. REPLACED THE LEFT FMC IAW AMM 34-61-02-000-801 AND AMM 34-61-02-400-801. PERFORMED CROSSLOAD OF FMC IAW AMM 34-61-00-470-806. COMPLETED FMC PERFORMANCE FACTORS ADJUSTMENT IAW AMM 34-61-00-800-801, AND PERFORMED FMCS OPS TEST IAW AMM 34-61-00-710-801. ALL TESTS AND OPS CHECKS PASSED.