FLC VISUALLY DETERMINES ILS GUIDANCE OF CTRLINE 1-2 MI.

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I WOULD LIKE TO BRING TO YOUR ATTN A VERY SERIOUS SAFETY CONCERN THAT I HAVE REGARDING FLYING INTO MEXICO CITY AND A SUBSEQUENT ERRONEOUS ILS SIGNAL FOR RWY 5R. SINCE THIS IS NOT THE FIRST TIME THAT I HAVE NOTED PECULIARITIES FLYING INTO MEXICO CITY ON THIS SAME APCH; I FIND IT NECESSARY TO RPT THE FOLLOWING INCIDENT. ON SEP/XX/95 I WAS THE FO OPERATING AND THE PF; FROM HOUSTON TO MEXICO CITY. THE WX WAS RPTED CLR; WITH 6 MI VISIBILITY. THE ACFT WAS A B737-500. UPON ARR INTO THE MEXICO CITY AREA; OUR FLT WAS CLRED FOR THE MATEO 5 ARR AND ILS APCH FOR RWY 5R. UPON REACHING THE MATEO VOR I SWITCHED FROM LNAV TO MANUAL VOR AND TRACKED S OF MATEO ON THE 160 DEG RADIAL. UPON REACHING THE 9.2 DME FROM MATEO; I SWITCHED OVER TO THE IMWX LOC FREQ 109.1 AND 052 DEG COURSE INBOUND. I IMMEDIATELY HAD AN INDICATION OF INTERCEPTING THE LOC AND WAS WITHIN 1 DOT OF SHOWING ON CTRLINE WITH THE RWY. I COMPLETED THE TURN TO THE INBOUND COURSE AND STARTED TO DSND ON THE GS. THE CAPT NOTICED THAT THE ARPT WAS APPROX 1-2 MI OFF TO THE R OF THE ACFT. I POINTED OUT THE ILS INDICATION THAT WAS ERRONEOUSLY SHOWING THAT THE ACFT WAS ON COURSE. I THEN CORRECTED THE COURSE AFTER VISUALLY SEEING THE RWY AND MADE A NORMAL LNDG. AT THE POINT I SELECTED AND TUNED THE ILS FREQ (9.2 DME S OF MATEO) I SHOULD HAVE HAD A FULL SCALE DEFLECTION OF THE LOC; NOT AN ERRONEOUS SIGNAL SHOWING THE ACFT TO BEING CLOSE TO CTRLINE ON THE APCH. THERE IS NO TERRAIN BTWN THAT POINT AND THE RWY THAT WOULD EXPLAIN A POSSIBLE WEAK OR INACCURATE SIGNAL. IT IS MY OPINION THAT THE PROB WITH THE LOC SIGNAL; IS GND BASED AND HAS NOTHING TO DO WITH ANY OF OUR ACFT. SOMEONE NEEDS TO INVESTIGATE AND INSURE THE RELIABILITY OF THIS SIGNAL TO AVOID THE VERY REAL POSSIBILITY OF A SERIOUS INCIDENT. PLEASE FEEL FREE TO CONTACT ME FOR ANY OTHER INFO. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR SAID HE WAS FLYING THE B737-500 WITH A COMPLETE GLASS COCKPIT; 2 IRS'S; 2 FMC'S; AND 2 CDU'S. (THE OLDER B737-300 AND 400 WITH ROUND DIALS HAD 1 FMC PORTED TO THE 2 CDU'S). THE RPTR BELIEVES THE PROB MAY BE A FALSE LOC (LIKE AT TORONTO AND BRUSSELS). WHY HE GOT THE FALSE LOC AND OTHERS DIDN'T HE AGREED WAS PUZZLING. HE DID SAY ON A SUBSEQUENT TRIP THE CTLR TOLD HIM HE WOULD CALL THE TURN ON TO THE ILS; SO; OTHERS MAY BE HAVING A PROB. BEING CAT II ILS EQUIPPED; THIS AIRLINE B737 EQUIP MAY HAVE AN INCREASED SENSITIVITY TO FALSE LOC SIGNALS THAT OLDER CAT I EQUIP DOES NOT HAVE. HE WISHES AN FAA ILS STANDARDS CHK WERE FLOWN ON THIS ILS AT MEXICO CITY. HE SAID HE HAS FLOWN THE LOC BENITO JUAREZ RWY 05R ILS 30-40 TIMES DURING HIS CAREER. POSSIBLY 5-8 OF THE RECENT FLTS HAVE HAD UNUSUAL ILS INDICATIONS. FOR EXAMPLE; ONE TIME HE WAS APCHING THE ILS LOC COUPLED TO THE AUTOPLT. THE AUTOPLT CAPTURED THE LOC BUT THERE WAS NO SYMBOL GENERATED LOC AND NO ILS AURAL IDENT UNTIL ON CTRLINE. THE ABSENCE OF THE SYMBOL GENERATED LOC IS THE SAME AS A LOC INVALID FLAG. HE SAID IT IS TRUE IF BOTH NAV RECEIVERS ARE MANUALLY TUNED IN RAW DATA FOR MORE THAN 12 MINS THE FMC WILL FLASH A WARNING 'IRS NAV ONLY' BUT THIS DID NOT HAPPEN. WHEN QUESTIONED HE RESPONDED THERE WERE NO PAX ELECTRONIC DEVICES BEING OPERATED IN THE CABIN AS THE AIRLINE FLT ATTENDANTS ARE ESPECIALLY CONSCIENTIOUS ABOUT THAT ON THIS AIRLINE. THE ANALYST SUGGESTED NEXT TIME HE MIGHT TURN ON HIS STANDBY ILS WHICH WOULD BE THE ONLY ILS LOC INDICATION INDEPENDENT OF THE SYMBOL GENERATOR OTHER THAN THE AUTOPLT ILS COUPLER. HE THOUGHT THAT WOULD BE A GOOD IDEA.