B737 FLC ISSUED RADAR VECTOR HDG TO PROCEED DIRECT ISO VORTAC WHEN ABLE. FLC OBSERVED SOUTHERLY DRIFTING HDG; ASSUME CAUSED BY WINDS. ATC INTERVENED; ISSUED VECTOR TOWARDS ISO VORTAC. COURSE DEFLECTION BEGAN APPROX 135 NM. AT APPROX 60 NM; SIGNAL STRENGTH WAS SUITABLE FOR NAV.

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IN THE VICINITY OF SNOW HILL VOR; WE WERE GIVEN A CLRNC OF '20 DEGS L; DIRECT KINSTON.' THAT CLRNC PUT US ON A 228 DEG HDG. WHEN WE GOT WITHIN ABOUT 145 NM OF KINSTON VOR; WE HAD GOOD AZIMUTH AND DISTANCE INFO. I CTRED THE CDI NEEDLE AND BEGAN TO TRACK INBOUND ON THE 223 DEG COURSE. AT ABOUT 110-120 NM; I THOUGHT WE WERE EXPERIENCING A WIND SHIFT. IT APPEARED AS THOUGH THE WIND WAS BLOWING OUT OF THE E; AND WE WERE DRIFTING TO R OF THE 223 DEG COURSE INBOUND. I GRADUALLY CORRECTED OUR HDG TRYING TO TRACK INBOUND ON THE 223 DEG COURSE. I CORRECTED TO APPROX A 195 DEG HDG. AT ABOUT 90 NM FROM KINSTON; ZDC CALLED US AND SAID THAT WE WOULD TRACK OUT TO SEA ON OUR PRESENT HDG. ZDC GAVE US A VECTOR AWAY FROM ANOTHER AIRPLANE AT OUR ALT AND A NEW HDG TO KINSTON. ZDC SAID THAT WE REACHED MINIMUM SEPARATION ON THE OTHER ACFT. ZDC ALSO TOLD US THAT KINSTON VOR PRODUCES UNRELIABLE SIGNALS IN THE VICINITY OF NORFOLK; VA; AND THAT WE SHOULD NEVER HAVE BEEN ISSUED A CLRNC FOR DIRECT KINSTON VOR N OF NORFOLK; VA. THE CAPT AND I CHKED ALL OF THE NOTAMS THAT WE HAD AVAILABLE AND COULD NOT FIND ANYTHING ABOUT THE UNRELIABILITY OF KINSTON VOR IN THE VICINITY OF NORFOLK; VA. NEITHER OF US HAD EVER HEARD OF THIS PROB. I THINK THAT PLTS SHOULD BE MADE AWARE OF THIS PROB. PLTS NEED TO KNOW WHEN THEY MAY RECEIVE A CLRNC TO KINSTON VOR; AND WHEN THEY MAY RELIABLY NAV TO KINSTON VOR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR CONFIRMED THERE WAS NEVER FULL NEEDLE DEFLECTION; JUST 2-3 DEG MOVEMENT EVERY FEW MI. BECAUSE THERE WAS SLOW DEFLECTION; NEITHER PLT BECAME CONCERNED UNTIL ALERTED BY ATC. PLTS ASSUMED DEFLECTION CAUSED BY WINDS. DEFLECTION BEGAN AROUND 135 MI. AFTER ATC VECTOR; SIGNAL STRENGTH ESTABLISHED AROUND 60 MI. ACFT WAS NOT NAV CTLED OR GLASS DISPLAY. MAINT SPECIALIST HAD NO RPTED PROBS WITH ISO VORTAC. NAVAID RECEIVED SCHEDULED FLT CHK WITHIN CERTIFICATION TIME FRAME (LAST 15 MONTHS). SPECIALIST VERIFIED THAT ISO WAS A LOW ALT NAV VORTAC; WITH CERTIFIED SVC VOLUME LIMITATION OUT TO 40 NM.