FLC OF MLG ACR ACFT DEVIATED FROM ASSIGNED TRACK DUE TO A MALFUNCTIONED NAV COMPUTER.

Narrative

OCEANIC XING AMSTERDAM-GANDER (MLG CPR JET) CLRED TO FL430 XING FROM EHAM-CYQX. PROGRAMMED FLT PLAN INTO GNS STO AMSTERDAM WITH ASSOCIATED LAT/LONG COORDINATES. UNEVENTFUL XING UNTIL LAST FIX; 51.00.0N X 050.00.0W. ESTABLISHED VHF COM WITH GANDER CTR AND RPTED 51.00.0N AND 050.00.0W. CTLR ASKED ACFT VERIFY POS UPON RADAR. SAID WE WERE AT 50N INSTEAD OF 51N. RE-VERIFIED POS AT TIME AND WE WERE AT 50N INSTEAD OF 51N AS CLRED. ACFT HAD JUST HAD VLF CTL HEAD REPLACED WITH UPDATED VERSION AT BEGINNING OF FLT 1 WK PRIOR. WE HAD DIFFICULTY DURING 6 PREVIOUS FLTS WITH VLF AND GPS GOING TO BR REPEATEDLY AND WERE TAKING ACFT BACK TO MAINT AFTER COMPLETION OF FLT TO HAVE UNIT LOOKED AT. UPON SUBSEQUENT PROGRAMMING AND CHKING OF UNIT; POS ENTERED SHIFTED 2 OF 3 TIMES ENTERED; AND WAYPOINTS MOVED AFTER PROGRAMMING. UNIT AND PROCESSOR WAS REMOVED BY MANUFACTURER FOR TEST AFTER SEVERE PROBS WERE SUBSEQUENTLY FOUND WITH INSTALLATION. SUGGESTION: RE-VERIFY FIX AND POS WITH OTHER STAND ALONE UNITS IN ACFT. IN THIS INSTANCE THE GNS/GPS INSTRUCTION HAD 2 STAND ALONE INS UNITS SLAVED TO IT FOR PROGRAMMING RTE FIXES.