ACR HAS TCASII RA AT SAME TIME AS ERRATIC FLUCTUATIONS ON LOC NEEDLE. ATC GIVES IMMEDIATE R TURN.

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DURING APCH; WE RECEIVED A TA ON TCASII INDICATING TFC AT 3 O'CLOCK AND ABOVE. WE WERE JUST BEGINNING TO ENTER IMC. CAPT WAS FLYING THE AIRPLANE AND I WAS LOOKING FOR TFC. WE BOTH THEN NOTICED ERRATIC FLUCTUATIONS ON LOC NEEDLE. SHORTLY AFTER; WE RECEIVED AN RA ON TCASII COMMANDING DSCNT AND ALMOST IMMEDIATELY WERE INSTRUCTED BY ATC TO TURN L IMMEDIATELY. CAPT PROMPTLY RESPONDED AND I GOT A QUICK GLANCE OF A JETLINER IN THE UPPER R COCKPIT WINDOW. WE THEN ENTERED THE CLOUDS. THE LOC INDICATIONS THEN RESUMED NORMAL OP. WORKLOAD WAS HVY AT THIS TIME. I XCHKED BOTH ILS INDICATIONS AND BOTH APPEARED TO GIVE NORMAL INDICATIONS. WE LANDED NORMALLY. AFTER PARKING AT THE GATE; I ASKED THE CAPT IF HE THOUGHT WE SHOULD CONTACT ATC BY TELEPHONE TO RPT AND DISCUSS INCIDENT. HE SAID NO. HE THEN EXPLAINED TO ME THAT THE ILS RECEIVERS ON THE PARTICULAR ACFT WE WERE FLYING HAD A HISTORY OF PRESENTING FLUCTUATING SIGNALS FROM TIME TO TIME BUT THAT THE PROB HAD NEVER BEEN FULLY RECTIFIED THOUGH ATTEMPTS WERE MADE. I WAS UNAWARE OF THIS. THE CAPT DID RPT THE LOC FLUCTUATIONS TO THE AVIONICS SHOP. I DO NOT KNOW AT THIS TIME WHAT CORRECTIVE ACTION; IF ANY; HAD BEEN TAKEN. IT WAS DIFFICULT TO RECALL DETAILS SINCE WORKLOAD WAS HVY; BUT I BELIEVE THE ERRONEOUS SIGNALS GIVEN BY THE LOC NEEDLE CAUSED US TO DRIFT TO THE R OF COURSE.