ALTDEV EXCURSION FROM ASSIGNED.

Narrative

WHILE DSNDING TO 5000 FT ON DSCNT INTO DFW; AUTOPLT ON; WE WERE INSTRUCTED TO SLOW TO 210 KIAS. I ASKED THE CAPT TO EXTENT THE SLATS. I DIALED IN 210 KTS IN THE SPD WINDOW. I THEN SELECTED 210 KIAS IN THE VERT SPD/IAS WINDOW. AT THE SAME TIME; THE ACFT LEVELED AT 5000 FT. THE CAPT RPTS THAT HE SAW 'ALT HOLD' IN THE FMA. SOMEHOW; POSSIBLY WHILE I WAS CHANGING THE SPDS; IAS WAS SELECTED IN THE FMA AND THE PWR WAS STILL BACK AT IDLE. THE ACFT LEFT 5000 FT IN A DSCNT. THE CAPT; NOTICING IAS IN THE FMA; SAID 'WHERE ARE YOU GOING?' REALIZING THAT THE ACFT WAS DSNDING; I IMMEDIATELY PUSHED 'ALTHOLD.' I THEN ROLLED THE VERT SPD WHEEL TO START A CLB. THE VERT SPD WINDOW SHOWED '1000 FT' AND I CONTINUED TO ROLL THE WHEEL NOSE UP. AT THAT TIME; THE ACFT WAS 200 FT LOW. THE ACFT WAS STILL DSNDING WHEN THE CAPT TOOK CTL; DISCONNECTED THE AUTOPLT AND CLBED BACK UP TO 5000 FT. NOT REALIZING THAT HE HAD RELINQUISHED CTL BACK TO ME; THE ACFT SANK AGAIN TO 200 FT BELOW OUR ASSIGNED ALT. I REENGAGED THE AUTOPLT; COMMANDED A CLB; AND LEVELED OFF AT 5000 FT. DURING THE FIRST ALTDEV; THE APCH CTLR REMINDED US TO STAY AT 5000 FT. WE RESPONDED THAT WE WERE THEN LEVEL AT 5000 FT. MAX DEV WAS 260 FT.