ALTDEV ALT OVERSHOT IN DSCNT.

Narrative

WE WERE USING THE FMS IN THE VNAV MODE TO DSND ON THE LAX 'CIVET 1 ARR.' SHORTLY AFTER BEGINNING DSCNT WE NOTICED THE CAPT'S FMS (CDU) SHOWING US APPROX 700 FT HIGH; THE FO'S DISPLAY SHOWED ONLY 40 FT HIGH. WE COMMENTED ON THIS THEN SAW THE CAPT'S DISPLAY (CDU) BEGIN TO RAPIDLY COUNT DOWN UNTIL IT AGREED WITH THE FO'S CDU. ALT WAS APPROX FL310; WITH BOTH COMPUTERS IN SYNC; WE CONTINUED THE VNAV DSCNT. DURING THE DSCNT PITCH CTL WAS NOT AS SMOOTH AS NORMAL; YET THERE WERE NO WARNING OR ERROR MESSAGES AS WE CONTINUED TO MONITOR THE DSCNT. THE ACFT MADE THE FIRST XING RESTR (CIVET). APCHING THE NEXT FIX (BREMR) THE AIRSPD BUG RAN AWAY FROM THE COMMANDED 330 KTS; TO 190 KTS. HOWEVER THE RATE OF DSCNT SEEMED TO INCREASE. AT THIS SAME TIME THE ATC CTLR REQUESTED WE SLOW TO 250 KTS. WHILE TRYING TO DETERMINE WHY THE AIRSPD BUG RAN AWAY AND REPROGRAM FOR 250 KTS; WE NOTICED THE ACFT DSNDING THROUGH 12000 FT; THE ALT RESTR AT BREMR. WHY THE VNAV MODE BUSTED THE ALT IS UNKNOWN TO US. THE AUTOPLT/VNAV WERE DISCONNECTED AS THE ACFT LEVELED OFF AT 11700 FT. THE CTLR WARNED US WE WERE BELOW THE RESTR ALT. AFTER CORRECTING THE ALT WE CONTINUED TO LAX. I BELIEVE WE HAD A FAILURE IN THE FMS/VNAV SYS; COMBINED WITH A HIGH WORKLOAD AT A KEY FIX PREVENTED ME FROM MONITORING THE ALT AS CLOSELY AS WE SHOULD HAVE.