XING ALT RESTR NOT MET. ALT UNDERSHOOT.

Narrative

OUR ACFT WAS GIVEN CLRNC TO CROSS SOMTO INTXN AT 13000 FT; NO SPD RESTR. DSCNT WAS BEGUN ACCORDING TO VERT GUIDANCE PLANNING INFO GENERATED BY THE ACFT (AIRBUS A- 320). DSCNT WAS BEGUN FROM 26000 FT. SOMETIME DURING THE COURSE OF DSCNT; NEARING 15000 FT; THE VERT SPD MODE ASSUMED CTL OF THE ACFT'S VERT DSCNT PLAN AND CAUSED IT TO GO OFF PROFILE ON THE HIGH SIDE; SUCH THAT THE XING RESTR WOULD NOT BE MADE UNLESS PLT INTERVENTION HAD BEEN MADE. WE DISCOVERED THE ERROR NEARING 14000 FT; AT APPROX 2 MI FROM SOMTO INTXN. UPON ATTEMPTING TO CORRECT THE DSCNT PROFILE; WE WERE QUERIED BY APCH CTL AS TO WHETHER WE WOULD MAKE THE XING RESTR. WE ACTUALLY CROSSED SOMTO INTXN ABOUT 500 FT HIGH. NO OTHER CTLR INTERACTION WAS REQUIRED AT THE TIME; AND WE WEREN'T AWARE NOR MADE AWARE OF ANY CONFLICTING TFC IN OUR IMMEDIATE (TCASII) VICINITY. THIS PROB UNDERSCORES THE NEED TO CONSTANTLY MONITOR THE PROGRESSION OF THE ACFT; ESPECIALLY WHEN IN THE MANAGED NAV (COMPUTER CTLED VERSUS PLT CTLED) MODE. AT THE TIME OF THE INCIDENT I WAS PREPARING TO MAKE A PA TO THE PAX; WITH DEST WX; ETC. THIS CHORE SHOULD HAVE BEEN ASSUMED BY THE PNF; BUT ON THIS PARTICULAR TRIP IT WAS BRIEFED THAT THE PF WOULD MAKE THE PA ANNOUNCEMENTS.