A320 FLT CREW HAS ALTDEV DURING 'PDZ 4' ARR TO LAX.

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ON MAR/FRI/03; I WAS SERVING AS A FO ON AN AIRBUS 320 WITH APPROX 150 PAX PLUS 5 CREW MEMBERS; FROM PHX TO LAX. AS WE APCHED LAX; APCH CTL CLRED US TO 'DSND VIA THE PDZ RWY 24R' APCH. WHEN WE FIRST RECEIVED THE RWY CHANGE FROM ATC; WE WERE AT 17000 FT BTWN TRTLE AND RIFFT INTXNS DOING APPROX 300 KIAS. THE XING ALT AT RIFFT INTXN IS 'AT OR ABOVE 14000 FT.' SINCE I HAD INITIALLY ENTERED RWY 25L IN THE FMGS ON THE GND IN PHX; A CHANGE IN RWYS WAS NOW REQUIRED. THE CAPT WHO WAS THE DESIGNATED PF SELECTED 'HDG' AND 10000 FT IN THE FCU RESPECTIVE WINDOWS AND BEGAN AN 'OPEN DSCNT' BECAUSE THE ACFT WILL NOT BEGIN A MANAGED DSCNT WITH 'HDG' SELECTED. HE SELECTED 10000 FT BECAUSE WE WERE SLIGHTLY HIGH ON THE ARR AND NEEDED TO BEGIN A DSCNT TO PLACE OURSELVES IN A PROPER VERT POS ONCE THE SECONDARY APCH WAS ACTIVATED. HE STARTED A DSCNT TO 10000 FT BECAUSE THE INTXN THAT INTERSECTED THE RWY CTRLINE LISTED A XING ALT OF 10000 FT. THE CAPT THEN SELECTED THE SECONDARY FLT PLAN PAGE ON THE FMGS COMPUTER (MCDU) TO SELECT RWY 24R; WHICH I HAD ENTERED ON THE GND IN PHX IN ANTICIPATION OF A RWY CHANGE IN LAX. THE CAPT STRUGGLED TO ACTIVATE THE SECONDARY FLT PLAN MAKING SEVERAL ATTEMPTS WITH NO SUCCESS. MEANWHILE; I FAILED TO MAINTAIN AWARENESS OF THE NOW RAPIDLY UNWINDING ALTIMETER IN 'OPEN DSCNT.' I WAS WATCHING THE CAPT AS HE WAS STRUGGLING WITH ACTIVATING THE SECONDARY FLT PLAN. WHEN HE FINALLY ACTIVATED THE FLT PLAN THE DISPLAY WAS NOT CORRECT. IT SHOWED THE ARR PROPERLY; BUT ALSO A SECOND GREEN LATERAL FLT PLAN LINE OFF OF RIFFT INTXN GOING TO THE SW AWAY FROM THE PROPER FLT PATH. HE MANAGED THE NAV MODE; BUT THE FLT BEGAN DRIFTING OFF OF THE COURSE. I MENTIONED THE IRREGULARITY OF THE UNUSUAL GREEN LATERAL LINE TO THE CAPT WHO THEN SELECTED DIRECT RIFFT ON THE MCDU AND THE ACFT PROCEEDED TO THE INTXN PROPERLY. ONCE HE HAD COMPLETED THE SECONDARY ACTIVATION; HE DID NOT IMMEDIATELY MANAGE THE DSCNT; WHICH I DIDN'T NOTICE UNTIL LATER. ABOUT THIS TIME WE BEGAN A DSCNT CHKLIST; BUT I HAD TO RE-ENTER THE LNDG DATA INTO THE LNDG PERFORMANCE PAGE ON THE MCDU; WHICH I HAD PREVIOUSLY ENTERED ON RWY 25L. AFTER I MADE THE ENTRIES WE CONTINUED WITH THE CHKLIST. DURING THIS TIME ATC GAVE US 2 AIRSPD CHANGES FOR THE ARR. ABOUT THIS TIME I NOTICED THAT THE MANAGED VERT DSCNT 'DONUT' RAPIDLY CAME FROM THE BOTTOM OF MY PRIMARY FLT DISPLAY MOVING UPWARDS. SINCE THE ACFT WAS STILL IN 'OPEN DSCNT' I MENTIONED TO THE CAPT THAT HE SHOULD SELECT MANAGED DSCNT; WHICH HE DID. HOWEVER; THE MANAGED DSCNT PATH (THE DONUT) WAS NOW ABOVE US AND THE AIRBUS FMGS IS NOT DESIGNED TO AGGRESSIVELY CAPTURE THE DSCNT PROFILE FROM THE BOTTOM OF THE PROFILE DSCNT PATH AND STRIVES TO MAINTAIN A SMOOTH 1G TRANSITION FROM 'OPEN DSCNT' TO 'MANAGED DSCNT.' WHEN 'MANAGED DSCNT' IS SELECTED FROM 'OPEN DSCNT' THE AIRBUS WILL SLOW ITS DSCNT IN THE MANAGED MODE TO 1000 FPM UNTIL IT CAPTURES THE PROFILE ALT. AT THIS POINT WE BOTH FAILED TO MONITOR THE DSCNT AND THE CAPTURING OF THE MANAGED DSCNT PROFILE. FURTHERMORE; SINCE THE CAPT HAD SELECTED 10000 FT IN THE ALT WINDOW OF THE FCU THE FLT CONTINUED DOWN REGARDLESS OF XING RESTRS THAT WERE NOW ABOVE IT; WHICH NEITHER OF US NOTICED. AT LEVELOFF AT 10000 FT; THE CAPT IMMEDIATELY RECOGNIZED WE WERE 2000 FT BELOW WHERE WE SHOULD HAVE BEEN ON THE ARR. TEJAY INTXN WAS 2 MI AHEAD OF US NOW AT A XING ALT OF '12000 FT OR ABOVE.' ATC DID NOT MENTION THIS ALT DISCREPANCY TO US. THE CAPT BEGAN A CLB BACK TO 12000 FT; BUT ONLY MADE IT TO 11000 FT BEFORE XING TEJAY INTXN. WE THEN SUCCESSFULLY RE-ENTERED THE DSCNT PROFILE AND CONTINUED WITHOUT FURTHER INCIDENT. SHORTCOMINGS OF PLT PERFORMANCE NOTED HERE INCLUDE: 1) THE CAPT'S INADEQUATE HANDLING OF THE DSCNT AND THE SELECTION OF THE SECONDARY FLT PLAN. 2) THE FO'S LOSS OF SITUATIONAL AWARENESS OF THE ACFT'S ALT AND PROPER XING RESTRS OF THE ARR. THIS INCIDENT HIGHLIGHTS SEVERAL PROBS WITH AUTOMATION OF MODERN ACFT THAT INCLUDES: 1) UNCERTAINTY WHERE TO FOCUS ATTN WHEN MULTIPLE TASKS NEED TO BE COMPLETED.2) EFFECTIVE STRATEGIES TO DEAL WITH THE SIT ARE LIMITED BY MULTIPLE TASKS BEING PERFORMED DURING THESE HIGH LOAD PERIODS. 3) INCREASED WORKLOAD DURING HIGH WORK DEMAND PERIODS. 4) LOSS OF SITUATIONAL AWARENESS. 5) A DEFINITE INCREASE IN STRESS. 6) CONFUSION OF THE AUTOMATION BY THE USER DURING HIGH STRESS PERIODS.