EMB145 CREW REPORTS LOC OSCILLATIONS DURING RWY 9R ILS PRM AT ORD.

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CLRED FOR ILS RWY 9R ORD WITH PARALLEL ILS RWY 9L APCHS IN PROGRESS WITH APCH MONITOR ACTIVE. WX RPTED: 070/13 5SM -RA BR 007BKN 014OVC. RADIOS TUNED AND IDENTED WITH STRONG IDENT TO ORD ILS RWY 9R 111.1. FROM SOUTHERN DOWNWIND; ATC GAVE US 060 DEG HDG TO INTERCEPT ILS RWY 9R AND CLRED US FOR THE ILS RWY 9R APCH AT 4000 FT. IN HDG MODE FO (PF) SET HDG TO 060 DEGS AND ARMED APCH MODE WITH THE AUTOPLT ON AND COUPLED TO THE FLT DIRECTOR. AFTER LOC CAME ALIVE AND CORRESPONDING FLT DIRECTOR CAPTURE; BOTH CAPT AND FO LOC HSI NEEDLES STARTED DEVIATING BACK TO THE L WITH THE AUTOPLT MAKING CORRESPONDING CORRECTIONS TO THE L (IT WAS 'CAPTURED' AT THIS POINT). BOTH LOC'S THEN SWUNG FROM L OF CTR TO R OF CTR WITH FLT DIRECTOR COMMANDING AND AUTOPLT EXECUTING A ROLLING TURN BACK TO THE R; CHASING THE HSI. FO DISCONNECTED THE AUTOPLT TO PREVENT EXCESSIVE BANK AND HDG CHANGES; PLACED FLT DIRECTOR INTO HDG MODE AND TOOK UP APPROX A 100 DEG HDG. SIMULTANEOUS WITH AUTOPLT DISCONNECT AND HDG MODE SELECTION; ATC APCH MONITOR STATED THAT WE WERE L OF COURSE AND ISSUED A R TURN AWAY FROM RWY 9L TO A HDG OF 120 DEGS AND A CLB TO 5000 FT. WE WERE HANDED OFF TO APCH CTL FOR VECTORS TO ANOTHER RWY 9R ILS. WE RPTED LOC PROBS TO APCH AND HE INDICATED THAT NO OTHER ACFT HAD RPTED ANY ILS DEVS OR PROBS. WE WERE VECTORED FOR ANOTHER ILS RWY 9R APCH BUT THIS TIME ATC PROVIDED INCREASED SEPARATION BY PLACING US SOLO ON RWY 9R WITH NO PARALLEL RWY 9L TFC. SIMILAR SIMULTANEOUS LOC DEVS OCCURRED AT APPROX 10 DME OUTSIDE OF DEANA WITH DEVS CEASING AS WE APCHED DEANA. COMPLETED SECOND ATTEMPTED ILS RWY 9R WITHOUT FURTHER INCIDENT. UPON REACHING THE GATE; MAINT LOG ENTRY WAS MADE REPORTING REPEATED LOC DEVS. TCAS WAS ON AND ACTIVE AT TIME OF LOC DEV; AND NO TA OR RA WAS ISSUED. ATC RPTED NO SEPARATION CONFLICT. WE BELIEVE THIS EVENT OCCURRED AS A RESULT OF UNEXPLAINED AND REPEATED (BOTH APCHS) LOC NEEDLE SWINGS TO THE L AND R OF CTR WITH CORRESPONDING AUTOPLT CORRECTIONS WHICH INITIALLY CHASED THE FALSE LOC INDICATION TO THE N OF COURSE. OUR INST SCAN PROVIDED THE FIRST INDICATION THAT THE EVENT HAD OCCURRED ALONG WITH ABNORMAL AUTOPLT COMMANDED BANKING AND HDG CHANGES TO THE L AND R AS IT TRIED TO MAINTAIN THE ERRANT LOC INDICATION. SUBSEQUENT NOTIFICATION BY ATC THAT WE WERE N OF COURSE CONFIRMED THIS. WITH KNOWLEDGE OF SIMULTANEOUS APCHS IN EFFECT TO RWY 9L; BOTH FLT CREW AND ATC DIRECTED ACTIONS TO TURN THE ACFT TO THE S WERE INITIATED. UPON RECOGNITION THAT THE FLT DIRECTOR WAS COMMANDING EXCESSIVE BANK ANGLE AND HDG CHANGES TO ATTEMPT RECAPTURE OF THE ERRANT LOC SIGNAL; THE FO DISCONNECTED THE AUTOPLT AND BEGAN TO ROLL THE ACFT OUT OF THE BANK AND TOOK UP A HDG OF APPROX 110 DEGS. SIMULTANEOUSLY; THE CAPT SELECTED HDG MODE AT THE FO'S REQUEST AND SELECTED THE HDG WITH THE HDG BUG. CAPT THEN VERIFIED THAT THE DEVS WERE PRESENT IN BOTH PLT'S HSI INDICATIONS WITH AN INST XCHK. CAPT COMMUNICATED PROB WITH ATC WHO THEN ISSUED THE CLBOUT AND TURN INSTRUCTIONS. I SUGGEST THAT THE CAUSE OF LOC SWINGS AND ERRANT FLT DIRECTOR COMMANDS IN THE EMBRAER 145 BE IDENTED AND CORRECTED. THIS SEEMS TO BE A RECURRENT PROB SPECIFIC TO THE FLEET TYPE AND SEEMS TO OCCUR AT 12-15 DME FROM THE LOC AND THEN SUBSIDE CLOSER TO THE MARKER.