FLC OF A WDB ACR ACFT DEVIATED FROM ASSIGNED COURSE DUE TO INS NAVIGATIONAL ERROR.

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FROM MIA TO ATL. AFTER LEAVING MIA AND DIVERTING FOR WX USING HDG MODE; WE WERE CLRED 'DIRECT DUBLIN SINCA 1 ARR.' IN CRUISE (37000 FT); APCHING DBN; CTR ASKED IF WE SHOWED ON COURSE. THE FMS SHOWED US ON COURSE APCHING DBN AND THE 2 MIN ALERT LIGHT WAS FLASHING. WE CHKED THE DBN COORDINATES AND THEY WERE CORRECT. UPON SWITCHING TO RADIO WE REALIZED THAT WE WERE L OF COURSE. CTR GAVE US A R TURN AND TOLD US THAT WE WERE GOING TO PASS W OF DBN BY ABOUT 13 MI; INSTEAD OF PASSING OVER IT; IF WE HAD CONTINUED ON OUR COURSE. CTR THEN GAVE US 'DIRECT CANUK INTXN.' WE USED THE FMS TO GO DIRECT AND CHKED THE COORDINATES IN THE FMS WHICH WERE CORRECT. THE ACFT TURNED TOO FAR TO THE L. WE THEN WENT BACK TO RADIO AND INTERCEPTED THE 'SINCA 1 ARR' IN COORD WITH CTR. WHILE APCHING 'CANUK' WE CHKED THE FMS AND IT INDICATED THAT WE WERE R OF COURSE 7 MI. #1 AND #2 INS INDICATED THAT WE WERE OFF COURSE. HOWEVER; #3 INS AGREED WITH RADIOS THAT WE WERE ON COURSE; WE CONTINUED INBOUND AND LANDED IN ATL. WE ASKED THE FLT ATTENDANTS IF ANYONE IN THE CABIN WAS USING SOME TYPE OF ELECTRONIC EQUIP. THEY SAID THAT THERE WERE SEVERAL PEOPLE PLAYING ELECTRONIC GAMES.