Air Carrier inbound to MEM on the ILS 36R experienced a 'false lock' on the localizer course at Olive Branch.

Narrative

I am submitting this safety report at the request of Memphis Approach Control management as a follow-up to a phone conversation I had with them on arrival at MEM; after experiencing a false lock on to the localizer course at Olive Branch; but in the opposite direction; while being vectored to ILS final for 36R at MEM. A few (3-5) miles southeast of Olive Branch Airport; descending from 5;000 FT to 4;000 FT; direct to HADAN; with about a 270 degree heading; we were cleared to intercept the localizer for MEM 36R and descend to 3;000 FT. We selected direct HADAN and armed NAV; and immediately armed the localizer. About one minute later; while passing directly south of Olive Branch; the FGS commanded a left turn; away from MEM and the autopilot followed this command. At about 20 degrees of bank; I directed the First Officer to fly the aircraft back to course to HADAN. He pushed the heading hold button and then clicked off the autopilot to initially proceed manually to HADAN. I had seen this a few months earlier on a daytime arrival at MEM but assumed it to be a false lock on to some lobe of the MEM localizer signal due to ground traffic or something at that time. This time; we were being vectored for an ILS final to accomplish an auto land equipment check and I thought that the signal should have been protected or I should have been advised otherwise. I alerted Memphis Approach what had just happened and the First Officer reselected NAV to proceed to HADAN. MEM Approach asked us to call the manager of Memphis Approach Control on arrival and to file a NASA report to help them capture as much information as possible on a chronic problem with the localizer signal at Olive Branch causing such anomalies. We never saw the localizer course come alive on the PFDs; but the steering bars did command a left turn and the FGS remained in NAV; with LOC armed until selecting heading hold. We never got more than .1 nm left of the original direct course to HADAN and Approach Control had no issue with us or our actions whatsoever. Subsequently given a new vector to final and re-cleared for the ILS 36R at MEM which went uneventfully from this point. In conversation with an ATC manager post-flight; he asked me to relay a request to all Company pilots for notification to Memphis Approach Control of such anomalies immediately when they happen; as the FAA is trying to trap as much data on these events as possible. He also informed me that an FAA technician is immediately dispatched to Olive Branch in a truck to record as much info at these times as possible from the suspect equipment. The ATC manager also asked what the make of our ILS equipment was and I asked him to address this question to our Tech folks for the most correct answer(s). He also related to me that other users; particularly our company call signs; had been experiencing this anomaly so often as to become desensitized to these events and write them off as 'the same thing that happened when we arrived here last week'. We were in the clear and immediately noticed the aircraft turning; somebody else might not be so lucky on arrival in the weather some time until this gets fixed.