NON ADHERENCE TO ATC CLRNC DURING A HDG TRACK POS DEV. NAV ERROR.

Narrative

DEP OUT OF EWR; FILED FOR ACAPULCO. CLRNC WAS TO LANNA; J-48 FOOTHILLS; AS FILED. OUT OF 10000 FT; FE GAVE OFF RPT AND WAS REQUESTED FOR OUR PLAN TO GO NONSTOP TO ACAPULCO OR MAKE FUEL STOP IN NEW ORLEANS. AS THAT WAS TOO CLOSE TO CALL AT THE TIME; I TOLD HIM I WOULD FIGURE IT OUT AS SOON AS I COULD. ON CLBOUT; WITH FO FLYING; AND MYSELF WORKING RADIO COMS; WE WERE GIVEN SEVERAL VECTORS ENRTE TO; AND PAST LANNA. FINALLY; WE WERE GIVEN A HDG (I RECALL EITHER 200 DEGS OR 220 DEGS) TO INTERCEPT THE 059 DEG RADIAL OF POTTSTOWN; RESUME NORMAL NAV. I READ BACK THE CLRNC; SET MY OWN HDG BUG AND COURSE BAR; POINTED AT THE FO HDG BUG AND REPEATED IT TO HIM AS HE SET THE BUG. THEN STATED THE 059 DEG RADIAL 'THAT'S 239 DEGS INBOUND' AS HE SET HIS COURSE BAR. AT THE SAME TIME; I TUNED HIS VOR TO PTW (116.5). I NOTICED AT THIS TIME THAT WE WERE LESS THAN 5 DEGS FROM INTERCEPT. I THEN TURNED MY ATTN TO CALCULATING FUEL BURNS TO SEE IF WE COULD MAKE ACA NONSTOP. AT THE NEXT RADIO CALL; I LOOKED UP TO SEE THAT WE HAD FLOWN THROUGH THE COURSE. NEW YORK WAS CALLING TO GIVE US A 270 DEG HDG TO REINTERCEPT. ON ARR AT ACAPULCO; I LEARNED THAT THERE HAD BEEN A TFC CONFLICT AND THAT THE CTLR HAD MADE 2 PREVIOUS CALLS USING THE WRONG IDENT; THERE WAS NO TA OR RA ALERTS ON THE TCASII. THE ONLY WAY THAT I COULD HAVE PREVENTED THIS OCCURRENCE WOULD BE TO NOT COMPLETELY DIVERT MY ATTN FROM THE NAV; WHILE PERFORMING THE OTHER TASKS. SUPPLEMENTAL INFO FROM ACN 293564: WHEN I TUNED IN THE VOR I WAS CLOSE TO THE STATION AND THOUGHT WE WERE STILL ON A HDG TO INTERCEPT S OF THE STATION. ALSO; THERE WAS CONFUSION ON MY PART TO INTERCEPT THE RADIAL WHILE ACCELERATING NEAR THE VOR AND ALMOST PAST THE RADIAL.