AUTOCOUPLED ILS APCH FLC DIFFICULTY ABANDONING APCH FOR GAR.

Narrative

ACFT WAS ON APCH TO ILS RWY 19 AT SDF ARPT. FO WAS FLYING THE APCH WITH THE AUTOPLT COUPLED AT THE TIME. ACFT WAS ESTABLISHED ON THE LOC TO RWY 19 AND DSNDING TO FINAL APCH FIX ALT OF 2800 FT AND CLRED FOR THE APCH BY ATC. AT APPROX 3000 FT; THE ACFT ENTERED HVY RAIN. SIMULTANEOUSLY; ATC ADVISED THAT THE VISIBILITY AT THE ARPT HAD DECREASED TO 1/2 MI AND THAT THE LAST 2 ACFT TO MAKE THE APCH HAD GONE AROUND. CAPT ELECTED TO ABANDON THE APCH AND ADVISED ATC THAT WE WOULD LEVEL OFF. HE THEN DISENGAGED THE ALT SELECT WHILE THE FO ATTEMPTED TO ARREST THE DSCNT WITH THE AUTOPLT PITCH WHEEL. HOWEVER; THE ACFT CONTINUED TO DSND; AND THE CAPT NOTICED THAT THE AUTOPLT HAD APPARENTLY CAPTURED THE GS AND WAS TRACKING IT DOWNWARD. AT ABOUT THIS TIME ATC ADVISED THAT THE ACFT IMMEDIATELY AHEAD OF OURS HAD ACCOMPLISHED A SUCCESSFUL LNDG AND WANTED TO KNOW IF WE DESIRED TO CONTINUE THE APCH. THE CAPT RESPONDED NEGATIVELY AND ATTEMPTED TO DISENGAGE THE APCH COUPLER BY SELECTING THE AUTOPLT COMMAND SWITCH FROM 'ILS' TO 'HDG.' FO AGAIN ATTEMPTED TO USE PITCH WHEEL TO ARREST DSCNT; BUT THE ACFT FAILED TO RESPOND AND CONTINUED DSCNT. AT 2500 FT MSL; FO DISCONNECTED AUTOPLT AND MANUALLY INITIATED A CLB BACK TO 3000 FT. THE ACFT WAS VECTORED BACK FOR AN UNEVENTFUL APCH AND LNDG. POOR WX; MULTIPLE COMS; AND AN UNRESPONSIVE AUTOPLT CONTRIBUTED TO THE ALT EXCURSION. A MORE RAPID DISENGAGEMENT OF THE AUTOPLT MAY HAVE PREVENTED THE EXCURSION.