A B767 CREW EXPERIENCED A NAV EQUIP DEV WHILE ON A 33 MI FINAL.

Narrative

AS WE APCHED ARNES INTXN ON THE CIVET ARR; WE HAD A MAP SHIFT OF ABOUT 1/4 MI TO THE R OR N. THE AUTOPLT WAS ON AND IT WAS IN LNAV AND FLT LEVEL CHANGE DSNDING. THE AUTOPLT ROLLED INTO A R TURN TO FOLLOW THE NEW SHIFTED TRACK AS WE DETERMINED IF THE LOC WAS CORRECT OR THE MAP. AS THE LOC WENT 1 DOT R OF COURSE; WE TURNED OFF THE AUTOPLT AND TURNED BACK TO THE CTR OF THE LOC. WE BOTH TURNED TO THE ILS MODE TO COMPLETE THE CIVET WITH LOC AND DME ONLY. AS WE TURNED BACK; ATC SAID THEY SHOWED US TO THE R OF CTR AND ASKED IF WE WERE CORRECTING. WE SAID WE WERE. AS WE APCHED THE FAF; I CHKED THE MAP AND IT HAD CORRECTED BACK TO NORMAL. BUT I THINK IT REMAINED SHIFTED FOR ABOUT 2 MINS. AFTER LNDG; I CALLED THE APCH CTL ON THE TELEPHONE AND HE SAID IT HAD NOT CAUSED ANY CONFLICT. HE DID TELL ME THOUGH THAT ACR HAD A PROB WITH THEIR B757 A WHILE AGO. JUST AS THEY WOULD HIT THE ARNES INTXN; THEIR MAP WOULD SHIFT L AND THE AIRPLANES WOULD START A TURN TO THE L. WE ALSO TOLD A MECH AND PUT THIS INFO IN THE ACFT LOGBOOK.