CAPT OF AN MD88 OVERSHOT DSCNT STAR ARR FIX DUE TO INADVERTENTLY OVERRIDING THE ALT CAPTURE MODE OF THE AUTOPLT WHEN CORRECTING FOR AIRSPD DESIRED.

Narrative

ACR X WAS DSNDING TO CROSS LAMMA INTXN ON THE BITH07 ARR INTO MLB. THE CAPT (ME) WAS AT THE CTLS. I WAS USING VNAV FOR THE DSCNT. THE FMS WAS PROGRAMMED TO CROSS LAMMA AT 15000 FT AND 320 KTS. APPROX 500 FT PRIOR TO LEVEL OFF; THE SPD COMMAND BUG SLOWED TO 240 KTS; THE THROTTLES WENT TO IDLE; AND THE SINK RATE INCREASED TO APPROX 1800 FPM DOWN. I SELECTED VERT SPD ON THE MODE CTL PANEL; SELECTED 500 FPM DOWN; AND INCREASED THE SPD BUG BACK TO 320 KTS. HOWEVER; WHEN I DID THIS I INADVERTENTLY DISARMED THE ALT CAPTURE. APPROX 100 FT ABOVE LEVEL OFF THE FO SAID 'ALT CAPTURE ISN'T ARMED.' I ATTEMPTED TO LEVEL OFF USING VERT SPD; BUT THE ACFT WAS VERY SLOW TO RESPOND. FINALLY; APPROX 150 FT BELOW OUR ASSIGNED ALT; I DISCONNECTED THE AUTOPLT AND CLBED BACK TO 15000 FT. THE FO AND I ESTIMATED THE ACFT HAD DSNDED AS MUCH AS 250-300 FT BELOW 15000 FT DURING THIS MANEUVER. THE MAIN FACTOR HERE WAS; FIRST; MY FAILURE TO IMMEDIATELY NOTICE THE FACT THAT THE ALT CAPTURE MODE WAS OVERRIDDEN WHEN I SELECTED VERT SPD; AND SECONDLY; MY LESS THAN IMMEDIATE DECISION TO DISCONNECT THE AUTOPLT TO RETURN TO 15000 FT. ATC MADE NO COMMENT TO US; THERE WAS NO CONFLICT WITH OTHER ACFT.