FLC OF AN MD88 WAS ADVISED BY APCH CTLR THAT THEY WERE SHOWN W OF LOC FOR CVG RWY 18L AND WAS THE REASON THAT A 90 DEG TURN WAS GIVEN TO THEM. THIS WAS A COMPLETE SURPRISE TO THEM SINCE THEIR LOC NEEDLE WAS CTRED; ALTHOUGH 13 DME OUT. SUBSEQUENTLY ON THE GND; OTHER CREWS RPTED PAST DIFFICULTIES NOTED WITH THE SAME NAVAID.

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VECTORED TO FINAL ILS RWY 18L (CVG) OUTSIDE OF BAIRE INTXN (APPROX 14 DME AT 5000 FT MSL). CLRED FOR ILS RWY 18L APCH. LOC CAPTURE CONFIRMED WITH RAW DATA. BOTH NAVAIDS IDENTED. PASSING APPROX 4600 FT MSL; ON GLIDE PATH; WAS INSTRUCTED TO LEVEL AT 5000 FT MSL; WITH NO EXPLANATION; FOLLOWED SHORTLY BY A TURN TO 090 DEGS SAYING WE WERE 'WELL W' OF COURSE. THIS WAS A SURPRISE TO US! SUBSEQUENT APCH WAS UNEVENTFUL. UPON EXITING ACFT TALKED TO ONCOMING CREW AND THEY HAD EXPERIENCED LARGE FLUCTUATIONS IN THE RWY 18L LOC EARLIER. WROTE UP COMPASS SYS AS A CHK AND FOR INSURANCE. NOTHING NOTED ON TCASII AND NO INDICATION OF CONFLICT. 'ATTA BOY' TO APCH CTLR. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT HE SUSPECTS THAT THE CVG RWY 18L LOC IS GIVING PROBS SINCE HE HAD A GOOD IDENT AND STRONG SIGNAL. HE DID SAY THAT THE ACFT COMPASS SYS HAD EXPERIENCED MINOR HEADING PROBS ON EARLIER LEGS OF HIS FLT SO HE HAD WRITTEN UP THE SYS FOR MAINT TO EXAMINE. HOWEVER; SINCE OTHER CREW MEMBERS WITH OTHER ACFT HAD EXPERIENCED DIFFICULTY ALSO WITH THE SAME LOC; HE RPTED THE DIFFICULTY ALSO TO HIS COMPANY CHIEF PLT. HE ALSO HEARD THAT THE ARPT WAS SHUTTING DOWN RWY 18L FOR MAINT; BUT DID NOT KNOW IF THE ACTION WAS RELATED TO THE NAVIGATIONAL LOC AID.