MLG MISSES XING RESTR ON ARR DUE TO IMPROPER INPUT IN FMC.

Narrative

ON NANCI 5 ARR TO LGA. FLT PLAN TO INTERCEPT ARR OVER DUPONT VORTAC (DQO) AT 17000 FT. RECEIVED CLRNC TO CROSS SOMTO INTXN AT 13000 FT. CHANGED ALT IN MODE CTL PANEL TO 13000 FT AND XCHKED BTWN CAPT AND FO. THE INTXN (SOMTO) WAS ENTERED ON THE FMC WITH THE XING RESTR. THE ACFT WAS ON AUTOPLT IN 'LNAV AND VNAV' TO PROVIDE THE XING AT SOMTO. ATC (WASHINGTON CTR) THEN ISSUED 'SLOW TO 250 KTS;' WHICH WAS ENTERED IN THE SPD WINDOW OF THE MODE CTL PANEL. THIS DISCONNECTED THE 'VNAV' AND THE ACFT DID NOT BEGIN TO DSND ON SCHEDULE AS PROGRAMMED. (WHEN SPD CHANGE IS ENTERED; VNAV DROPS OFF AND ALT HOLD IS ON.) WE BEGAN DSCNT TOO LATE TO MAKE THE XING RESTR OF 13000 FT AT SOMTO. ATC ASKED ALT AND I MADE A RAPID DSCNT TO 13000 FT WHICH CAME SHORTLY AFTER SOMTO. ATC ADVISED NO TFC CONFLICT AND WE CONTINUED TO LGA. WHEN ALT AND AIRSPD CHANGES OCCUR; IT SHOULD BE ENTERED IN THE COMPUTER OF THE FMC; HOWEVER THE COCKPIT WORKLOAD IN NE BTWN WASHINGTON AND NEW YORK IS EXTREMELY HIGH. IF THE COMPUTER IS BEING USED FOR XING RESTRICTIONS; IT MUST BE RECHKED TO BE SURE A MODIFICATION HAS NOT REMOVED THE AUTOPLT FROM THE INTENDED MODE (VNAV OR LNAV.)