What was found
FLT#20, LHR-JFK. (ETOPS) "BOTH FMS UNITS INTERMITTENTLY DROP CRUISE ALTITUDES CAUSING INACCURATE TIME AND FUEL PREDICTIONS AT UPCOMING FIXES, DISPATCH SENT DIRECTIONS TO PRINT FLIGHT PLAN, DELETE SECONDARY AND RE INSERT CRUISE ALTITUDE THEN FCOMVI 5.2.7.17 AND NEITHER WORKED. WE ALTERED OCEANIC CONTROL, THEN PLUGGED IN CRUISE ALTITUDE AT UPCOMING FIXES SEVERAL TIMES IN ORDER TO GET ACCURATE TIME AND FUEL ACROSS NAT HLA; REMOVED AND REPLACED FMGC 1 IAW AMM 22-83-84 . OPS CHK GOOD . REF LOGPAGE 4297052-1(PERFORMED OPS TEST OF LAND CAT III CAPABILITY IAW AMM 22-97-00-710-001-A . TEST PASSED . A/C OK FOR CAT III) CAT II TEST PASSED."