B757 FLT CREW REPORTS ANOMALOUS AUTO PILOT OPERATION DURING ILS APPROACH TO RWY 8 AT TJSJ.

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LEVEL AT 1600 FT; WE WERE GIVEN A HDG TO INTERCEPT THE RWY 8 LOC AND CLRED FOR THE ILS TO RWY 8. FO WAS FLYING AND USED HDG SELECT TO GO FROM LNAV TO HDG CTL. HE ALSO SELECTED LOC. WE BOTH NOTICED THE HDG SELECT DID NOT RESPOND RIGHT AWAY AND THEN WAS VERY SLOW TO TURN TO ASSIGNED HDG. AS WE ARE TURNING TO INTERCEPT THE LOC; THE FO ARMED 'APP' AND A FEW SECONDS LATER THE AUTOPLT DISENGAGED UNCOMMANDED AND I NOTICED AN AMBER LINE THROUGH MY GS INDICATION ON MY ADI. THE NEXT THING I NOTICE IS THAT WE ARE NOW AT 1200 FT. FO CORRECTED BACK AND WENT ON TO AN UNEVENTFUL LNDG. POINTS: 1) THIS IS THE SECOND TIME SOMETHING VERY SIMILAR HAS HAPPENED TO ME AT THE SAME TIME ON THE SAME APCH. 2) THE GS WAS FULL SCALE UP WHEN THIS HAPPENED. IT VERY MUCH SEEMED LIKE WE INTERCEPTED A FALSE GS. THE FO SAID HE SAW A FLY DOWN COMMAND ON THE FLT DIRECTOR. 3) I HAVE ALTERED MY PROC FOR ARMING THE LOC/APCH PROC BASED ON MY LAST EXPERIENCE HERE; BUT THE FO HAD ALREADY ARMED 'APP' BEFORE I SAW IT AND THINGS WERE HAPPENING VERY FAST. 4) SOMETHING IS NOT RIGHT JUST OUTSIDE THE MARKER ON THE ILS RWY 8 APCH AT TJSJ.