A B737-300 FLC EXPERIENCES AN UNRELIABLE LOC DURING AN ILS APCH INTO JACKSON HOLE; WY. THE WX WAS FORTUNATELY VMC WITH THE ARPT AND RWY IN SIGHT DURING THIS APCH. ANOTHER ACFT VERIFIED THE LOC COURSE PROB. CREW RPTED THIS EVENT TO ZLC CTLR.

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CLRNC WAS ISSUED TO CONDUCT AN ILS APCH TO RWY 18 AT JAC ARPT FROM THE ENRTE AIRWAY SYS. AFTER THE TURN TO FINAL AT 12000 FT MSL; THE ILS FINAL APCH COURSE WAS COMMENCED. AFTER OPERATING IN LNAV TO INTERCEPT THE COURSE; GUIDANCE WAS SWITCHED TO THE ILS MODE FOR TRACKING AND DSCNT TO THE ARPT. AFTER TRACKING FOR A SHORT TIME TO THE FAF IT WAS DETERMINED THAT THE LOC WAS UNRELIABLE; BUT THE GS WAS RELIABLE. AFTER RPTING THIS CONDITION TO ZLC; THE APCH WAS CONTINUED VISUALLY (ARPT AND RWY IN SIGHT THROUGHOUT APCH) TO AN UNEVENTFUL LNDG. ZLC WAS SURPRISED TO HEAR ABOUT THE MALFUNCTION AND ASKED ANOTHER CARRIER TO MONITOR THE ILS ALSO. THIS CONFIRMED THE PROB WITH THE ILS. I SURMISE THAT THE APCH IS NOT MONITORED FOR THE PURPOSES OF NOTAMS. ADDITIONALLY; A NOTAM CHK AFTERWARD INDICATED NO PROBS WITH THE ILS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR WAS FLYING A B737-300. THE OTHER ACFT WAS AN FMC EQUIPPED ACFT. JAC; WY; IS A CTAF NON TWR ARPT. PIC HAD RPTED THIS PROB TO ZLC ONLY AND DOES NOT KNOW ANYTHING MORE ABOUT THIS SIT. HE STATED THAT HE WAS IN VMC CONDITIONS.