AUTOPLT ERRATIC DURING VISUAL APCH. PAX CELLULAR PHONE USE SUSPECTED.

Narrative

ON APCH TO ORD -- VFR CONDITIONS -- WE WERE AT 4000 FT HDG 090 DEGS AND CLRED TO INTERCEPT RWY 4R LOC. AUTOPLT WAS ARMED FOR INTERCEPT. AS THE COURSE CAME OFF THE WALL THE AUTOPLT SMOOTHLY STARTED TO INTERCEPT. ABRUPTLY; THE AIRPLANE ENTERED A 30 DEG BANK AND STARTED TO TURN AWAY FROM THE LOC. USING HDG SELECT; WE TURNED THE PLANE BACK TOWARDS THE LOC AND REARMED THE AUTOPLT FOR LOC INTERCEPT. AGAIN; THE AIRPLANE STARTED TO INTERCEPT THE COURSE AND THEN ABRUPTLY TURNED TO THE R; AWAY FROM THE LOC COURSE. THIS HAPPENED AGAIN A THIRD TIME. AUTOPLT WAS DISCONNECTED AND UNEVENTFUL VISUAL LNDG MADE. BOTH CAPT AND FO FLT DIRECTORS WERE CORRECTLY TUNED TO THE RWY 4R LOC FREQ AND THE AIRPLANE HAD NO MAINT HISTORY OF AUTOPLT PROBS. WE HAD A FULL LOAD OF PAX BUT; DUE TO THE PHASE OF FLT; WE DID NOT HAVE TIME TO INVESTIGATE THE CABIN FOR ELECTRONIC DEVICES. AFTER DEBRIEFING THE FLT ATTENDANTS AT THE GATE THEY INFORMED ME THE PAX IN 28D HAD A LARGE CELLULAR TELEPHONE THAT THEY SUSPECT HE WAS USING. THEY HAD REPEATEDLY TOLD HIM TO DISCONTINUE ITS USE DURING THE DSCNT. THIS INCIDENT MATCHES OTHER PUBLISHED RPTS OF ELECTRONIC INTERFERENCE -- AUTOPLT BANKING UNCOMMANDED AWAY FROM LOC COURSE.