AN ACR FLC EXPERIENCED DRIFT WHILE FAF INBOUND; ON AN ILS APCH. JUST PRIOR TO MINIMUMS; THE LOC RATE OF DEFLECTION CHANGE WAS SO SIGNIFICANT THAT A MISSED APCH WAS EXECUTED.

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AT THE TIME STL WAS USING RWY 30R FOR LNDG; AND TAKING OFF ON RWY 30L. PARALLEL APCHS WERE NOT BEING USED BECAUSE OF THE LOW CEILING. WE WERE CLRED FOR AN ILS APCH TO RWY 30 R WITH THE FO FLYING THE ACFT. IT IS COMMON FOR THE CAPT AND FO TO ALTERNATE LEGS; AND IT WAS HIS TURN. THE WX CONDITIONS WERE NOT BAD ENOUGH TO REQUIRE A MONITORED APCH: THEREFORE; THE FO WOULD FLY THE ENTIRE APCH AND LNDG. THE APCH WAS NORMAL UNTIL JUST INSIDE THE OM; AT WHICH POINT WE STARTED DRIFTING TO THE L. THE FO CORRECTED BACK TO THE LOC; BUT ON RETURNING TO THE ORIGINAL HDG WE DRIFTED VERY RAPIDLY TO THE L AGAIN. BY THIS TIME; SINCE WE WERE GETTING CLOSE TO MINIMUMS AND STILL IN THE CLOUDS; I HAD DECIDED THAT WE WOULD GAR. BEFORE I COULD VERBALIZE MY THOUGHTS THE FO SAID; 'WE HAD BETTER GAR;' (OR SOMETHING TO THAT EFFECT) AND I RESPONDED: 'GAR.' WHICH HE DID. STL VECTORED US AROUND FOR ANOTHER APCH AND WE LANDED ON RWY 30 R; WITHOUT INCIDENT. SUPPLEMENTAL INFO FROM ACN 298105: AS I PROCEEDED ALONG THE APCH; I STARTED TO DEV TO THE R (N) OF THE LOC. I BEGAN TO CORRECT TOWARDS THE LOC; AND IN SO DOING; RECAPTURED THE LOC. AT THIS POINT I BEGAN TO TRACK AWAY FROM THE LOC TO THE L (S) AT AN UNUSUALLY FAST RATE; EVEN GIVEN THE XWIND FROM THE R (350 DEGS AT 10 KTS AT A POINT BEFORE APCH MINIMUMS I ELECTED TO MAKE A MISSED APCH. THE LOC BEHAVED IN A MOST PECULIAR MANNER; IN ITS RATE OF MOVEMENT. WHETHER THIS ERRATIC LOC BEHAVIOR WAS A RESULT OF INTERNAL OR EXTERNAL NAV EQUIP FAILURE; CELLULAR PHONE USAGE; OR PLT INDUCED OSCILLATION; I CAN'T SAY; BUT SOMETHING WAS ODD; AND I BELIEVE A MISSED APCH WAS NECESSARY.