A B737-200 FLC DSNDED 300 FT BELOW ASSIGNED ALT ON DSCNT TO DAL.

Narrative

ACFT DEPARTED AUSTIN BERGSTROM ARPT AT XA15 ON SEP/XA/99 FOR DALLAS LOVE FIELD IN VMC. ON DSCNT INTO DALLAS; REGIONAL APCH CTL DIRECTED THE ACFT TO INTERCEPT THE RWY 31R LOC AND DSND TO 5000 FT MSL. DURING DSCNT (ON THE LOC) THE CAPT DIRECTED THE FLT ATTENDANTS TO SECURE THE CABIN FOR ARR ON INTERPHONE. THE '1000 FT TO GO' COMPANY ALT CALL WAS GIVEN AND ACKNOWLEDGED PASSING 6000 FT. AT APPROX 5300 FT MSL THE CAPT WAS OBSERVED DECREASING RATE OF DSCNT FOR LEVELOFF. THE COPLT THEN POSITIONED THE ACFT'S TRANSPARENT WINDSHIELD SUNSCREEN TO DAMPEN THE SETTING SUN GLARE; THEN NOTICED THE ALT PASSING APPROX 4800 FT AND NOTIFIED THE CAPT OF THE 5000 FT ALT REQUIRED ALT. THE CAPT RESPONDED BY IMMEDIATE CLB BACK TO 5000 FT. APCH CTL ALMOST SIMULTANEOUSLY CLRED THE ACFT TO 4000 FT AND A SUBSEQUENT VISUAL APCH TO RWY 31R. THERE WAS NO AIR TFC COLLISION; AVOIDANCE POTENTIAL NOR WAS THERE ANY TCASII TFC PROX INDICATION. HOWEVER; ALT DEVIATED FROM THAT ASSIGNED BY APPROX 300 FT. THIS WAS THE SECOND FLT OF THE DAY OF THE LAST DAY OF A 4 DAY PAIRING. THE 4 DAY TRIP HAD INCLUDED EXCELLENT CREW COORD; HOWEVER; THIS WAS THE FIRST FLT IN AN ACFT WITHOUT AUTOMATIC ALT LEVELOFF CAPABILITY IN AUTOPLT OP. POSSIBLY THIS DIFFERENCE AND PERFORMANCE OF OTHER COCKPIT ACTIONS DURING CRITICAL LEVELOFF LED TO THIS DISCREPANCY.