What was found
VTK APPEARED INTERMITTENTLY ON CAPTAIN'S NAV DISPLAY. NO FMC DATA AVAILABLE ON EITHER CDU. WHILE CONSULTING QRH, LEFT FMC BEGAN WORKING NORMALLY AGAIN. LEFT FMC FAILED AGAIN ON DESCENT, CONTINUED FLIGHT ON RIGHT FMC ONLY. ACFT GROUNDED. REMOVED, REPLACED, AND TESTED LEFT FMC COMPUTER IAW AMM 34-61-02-400-801, ALL TESTS PASS. RETURNED FMC SELECT SWITCH TO NORMAL POSITION.