ACR MLG ALT DEVIATION OVERSHOT DURING DESCENT INTO DFW.

Narrative

WE WERE APCHING DFW ON THE BOIDS 9 ARR WITH CLRNC TO CROSS PIVOT INTXN AT 11000' AND 250 KTS. IN DSCNT THE ACFT WAS ON AUTOPLT IN THE IAS MODE AND 11000' ARMED. AT 12500' ATC ASKED US TO EXPEDITE DSCNT. I SELECTED VERT SPD ON THE FLT GUIDANCE PANEL AND INCREASED OUR RATE OF DSCNT FROM 1500 FPM TO 2500 FPM. 11000' SHOULD HAVE REMAINED ARMED DURING THIS PROC. IT DID NOT AND I DID NOT NOTICE IT DROP OUT OF THE AUTOPLT ANNUNCIATOR UNTIL APPROX 11200'. THE ACFT WAS STILL IN A 2500 FPM DSCNT WHEN HIT ALT HOLD. THE ACFT DID NOT STOP ITS DSCNT UNTIL 10600'. I IMMEDIATELY CLBED AND LEVELED AT 11000'. THIS IS NOT THE FIRST TIME THE MLG AUTOPLT HAS DROPPED THE ARMED ALT IN MY EXPERIENCE; AND I SHOULD HAVE NOTICED IT IN THE ANNUNCIATOR. HOWEVER; WE WERE ON AN ARR 25 MI FROM DFW DURING A PEAK TFC PERIOD ON A VFR DAY. I HAD DIVERTED MY ATTN OUTSIDE TO WATCH FOR TFC FOR NO MORE THAN 30 SECS; WHICH WAS ENOUGH TO MISS THE ALT. THE ALT CAPTURE FUNCTION OF THIS AUTOPLT SHOULD BE REDESIGNED TO MAKE IT MORE DIFFICULT TO DISARM SO MORE ATTN CAN BE FOCUSED OUTSIDE.