DEVIATION FROM TRACK 20 MILES 130 MILES SOUTH OF VOR.

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COPLT'S LEG ON TRIP FROM SEATTLE TO ANCHORAGE. BOTH VOR'S TUNED TO BIORKA ISLAND VOR (BKA) AND TRACKING INBOUND ON J501 WITH AUTOPLT ENGAGED USING VOR-LOC MODE. NO 1 ADF TUNED TO SIT AND NO 2 TO ZP. INITIALLY THE AUTOPLT CAPTURED THE COURSE AND TRACKED NORMALLY FOR SEVERAL MINUTES. SHORTLY THEREAFTER I NOTICED THAT THE ACFT WAS IN A RIGHT BANK AND NO LONGER IN THE VOR LOC MODE AND TO THE RIGHT OF COURSE. I IMMEDIATELY MENTIONED THIS TO THE COPLT WHO STARTED MAKING CORRECTION TO THE LEFT AT WHICH TIME CENTER CALLED TO SAY WE WERE 20 MILES TO THE RIGHT OF COURSE. AT THIS TIME THE COURSE DEVIATION BAR WAS FLUCTUATING AS MUCH AS 6 TO 7 DEG LEFT TO RIGHT ALTHOUGH THE IDENT WAS STILL GOOD AND CENTER WAS ADVISED OF THIS. A FLT IN THE VICINITY ALSO MENTIONED TO CENTER THAT THEY HAD EXPERIENCED SIMILAR FLUCTUATIONS FOR OVER A YEAR. WHEN THE BKA VOR SETTLED DOWN AND WE HAD OUR COURSE DEVIATION BARS CENTERED WE CHECKED WITH CENTER AND WERE ADVISED WE WERE 6 MILES TO THE RIGHT OF COURSE. OUTBOUND FROM BKA WE CHECKED AGAIN WITH CENTER AND ALTHOUGH OUR COURSE BARS WERE CENTERED WE WERE 5 MILES TO RIGHT OF COURSE WHEN WE WERE 100 DME NW. INBOUND TO JOH AND ANC WE CHECKED OUR NAVAIDS AND EVERYTHING APPEARED TO BE NORMAL. CLOSER CHECKING OF OUR ADF'S AND AN EARLY CALL TO ANC CTR FOR A RADAR FIX WOULD HAVE ALLEVIATED MUCH OF THE PROBLEM.