A DSNDING MD90 DSNDS BELOW THE ALT XING AS DESIGNATED FOR KAYOH INTXN WHEN THE FMS MALFUNCTIONS AND DISTRACTS THE CREW FROM THEIR DSCNT PROC.

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WE WERE CLRED TO CROSS KAYOH AT 7000 FT AND PROCEED VIA EASTSIDE 3 ARR. THE CAPT WAS USING A VNAV DSCNT SETUP FOR THE EASTSIDE 3 FMS STAR AND VERIFIED BY BOTH PLTS. DURING OUR DSCNT TO 7000 FT THE CAPT NOTICED A MAP FAILURE AND ATTEMPTED TO UPDATE THE ACFT POS. I NOTICED OUR NAV DISPLAYS SHOWED THE RTE AS A DASHING BLUE LINE INSTEAD OF THE NORMAL PINK RTE LINE. THE AUTOPLT REMAINED ON IN THE 'HEADING HOLD' MODE AND VERT SPD. SINCE THE FMS WAS SET UP FOR THE STAR THE CAPT HAD RESET THE DIGITAL FLT GUIDANCE COMPUTER ALT TO 3300 FT FOR 'SNAKE INTXN.' I ATTEMPTED TO REGAIN OUR ARR SINCE WE WERE ON AN 'FMS STAR' WITH NO NAVAID BACKUPS. AFTER REACTIVATING THE RTE AND EXECUTING; THE NAV DISPLAY SHOWED A NORMAL DISPLAY. THE CAPT RE-ENGAGED NAV AND ATTEMPTED TO RE-ENGAGE VNAV. THE FMS DISPLAYED 'NO VERT NAV' AND 'NO PERFORMANCE DATA.' AS I WAS ENTERING THE PERFORMANCE DATA THE AUTOPLT AGAIN SWITCHED TO 'HEADING HOLD AND VERT SPD' MODE. AT THIS TIME ATC RADIOED ABOUT OUR ALT APPROX 5900 FT MSL. THE CAPT IMMEDIATELY DISCONNECTED THE AUTOPLT AND STARTED A CLB TO 7000 FT MSL. ATC ASKED IF WE WERE VMC; WHICH WE WERE; AND CLRED US TO PROCEED VISUALLY; MAINTAINING TERRAIN CLRNC AT 5000 FT OR ABOVE. THE REST OF THE FLT WAS UNEVENTFUL. TO PREVENT RECURRENCE I'LL NOTIFY ATC IMMEDIATELY WITH EQUIP FAILURES. ENSURE SOMEONE IS FLYING THE AIRPLANE WHILE THE OTHER PLT CORRECTS THE MALFUNCTION. PROVIDE A BETTER BACK UP FOR THE PF. SUPPLEMENTAL INFO FROM ACN 402691: I THEN NOTICED THE AUTOPLT HAD AGAIN GONE TO 'HEADING HOLD' AND WAS STILL IN 'VERT SPD' DSCNT AND I WAS 1200 FT OR 1300 FT BELOW 7000 FT AND 2 OR 3 MI E OF KAYOH. I KICKED OFF THE AUTOPLT AND STARTED A CLB BACK UP TO 7000 FT WHEN SOCAL CALLED AND SAID WE WERE CLRED TO DSND AND THEN SAID WE WERE TOO LOW AND TO CLB BACK TO...THEN HE ASKED IF WE WERE VFR. TO PREVENT A RECURRENCE; I'LL SET THE ALT WINDOW FOR THE BOTTOM ALT AT EACH WAYPOINT ON THE STAR.