UNACCEPTABLE GYRATIONS DURING A COUPLED ILS APCH INTO GEG; WA. MAP PERFORMED.

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ALL; UP TO LOC INTERCEPT AT GEG RWY 3; WAS NORMAL. CLRED ILS RWY 3 AT APPROX XX40 PM LCL. ILS WAS ARMED AT 5000 FT MSL; I BELIEVE; ON A VECTOR HEADING TO INTERCEPT ILS. I BELIEVE THAT THE ACFT INITIALLY CAPTURED THE LOC AND BEGAN A SERIES OF 'S' TURNS THROUGH THE LOC OUTSIDE OF THE OM. DRIFT FROM THE PMS WAS 9-10 DEGS L (R CRAB). GS CAPTURED AND FOR SOME REASON ACFT AUTOPLT THEN WENT TO AN ALT HOLD MODE. I BELIEVE; AT THAT TIME; WE WERE R OF LOC CTRLINE. I REARMED THE ILS AND BOTH LOC AND GS RECAPTURED. THIS OCCURRED JUST INSIDE THE OM I BELIEVE. ANOTHER SERIES OF 'S' TURNS ENSUED AT UP TO 30 DEGS OF BANK. AT APPROX 3400 FT MSL 1 DOT DEFLECTION L OF THE LOC; AND IN A 30 DEG R BANK; I INITIATED A MAP. ALL QUESTIONS TO TWR CONCERNING ILS CRITICAL AREA; ILS FUNCTION; ETC; INDICATED NORMAL OP. WE CLBED TO 6000 FT MSL AND WERE VECTORED AROUND FOR ANOTHER APCH. ON A 20-30 DEG INTERCEPT VECTOR WE WERE CLRED FOR THE ILS AGAIN; DRIFT WAS AGAIN 9-10 DEGS L. APCH THEN ASKED IF WE SHOWED OURSELVES ON LOC CTRLINE. WE SAID NO; AS CDI CASEBREAK HAD NOT YET OCCURRED. MY RESPONSE TO APCH WAS; WHY? DID HE SHOW US ON CTRLINE? HE SAID NO; THAT IT LOOKED LIKE WE WERE PARALLELLING THE LOC SOMEWHAT. LOC CAPTURED OUTSIDE THE OM AND WAS SLIGHTLY UNSTABLE; BUT PROBABLY NORMAL GIVEN THE AMOUNT OF DRIFT THAT WAS PRESENT; THE REMAINDER OF THE APCH WAS NORMAL AND WE BROKE OUT OF MINIMUMS AND LANDED ON RWY 3. MY DECISION TO NOT RPT VIA A COMPANY RPT WAS DUE TO THE FACT THAT WE; AT THIS TIME; HAVE NOTHING TO RPT; SINCE BOTH ACFT SYS AND ILS SYS OPERATED NORMALLY ON SUBSEQUENT APCH; AND ATC RPTED THAT ILS WAS NORMAL ON BOTH APCHS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: HE FIRMLY BELIEVES THAT THERE HAD BEEN AN INTRUSION INTO THE ILS CRITICAL HOLDING AREA. THE TWR WAS RPTING A 6000 FT RVR. THE ACTUAL RVR WAS 2000 FT. THE GND HOLDING POS MAY THEN HAVE BEEN VIOLATED BY HOLDING ACFT. THE TWR WAS REALLY NOT CAPABLE OF OBSERVING OTHER ACFT IN THE HOLDING AREA DUE TO THE REDUCED VISIBILITY AND 'SENT' SOMEONE DOWN TO CHK ON THAT AREA; BUT AFTER THE FACT. ANOTHER COMPANY ACFT HAD BEEN MAKING APCHS TO RWY 3/21 ALL DAY AT AN EARLIER TIME AND RPTED NO PROBS WITH THE ILS ITSELF. RPTR COULD NOT EXPLAIN THE OTHER ACFT EQUIP ANOMALIES THAT WERE EXHIBITED.