ACR WDB ALTDEV OVERSHOT DURING DSCNT INTO LAX ON CIVET PROFILE TRYING VERY HARD TO ANALYZE AUTO APCH RESPONSE TO PRESENT PROFILE.

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FLT WAS IN CRUISE AT FL390 INBOUND TO LAX. FMCS WAS PROGRAMMED FOR CIVET 2 PROFILE DSCNT RWY 25L APCH. AS ACFT NEARED DSCNT POINT ATC CLRED FLT TO FL350. THE CRUISE PAGE ON FMC WAS SELECTED AND FL350 ENTERED. ACFT STARTED DSCNT FURTHER CLRNCS TO FL310; FL290 AND FINALLY FL240 WERE ISSUED AND SPD REDUCTIONS TO 280 KTS AND 250 KTS WERE ALSO ISSUED. ACFT THEN CLRED FOR CIVET 2 PROFILE DSCNT. 8000 FT WAS SELECTED ON MASTER CTL PANEL. THIS WAS THE FINAL ALT IN THE DSCNT PROFILE ALL OTHER XING ALTS WERE ALREADY PROGRAMMED IN THE FMCS AND THEREFORE SHOULD HAVE BEEN MET BY THE ACFT SINCE THE SPD OF 250 KTS WAS ISSUED BY ATC SPD INTERVENTION WAS SELECTED AND ACFT DSNDED. AS ACFT NEARED 10000 FT I DESELECTED SPD INTERVENTION BUT INSTEAD OF 250 KTS BEING SELECTED THE ACFT BUG WENT TO 320 KTS. I RESELECTED SPD INTERVENTION AND 250 KTS. WHEN ACFT LEVELED AT 10000 FT NEAR ARNES I DESELECTED SPD INTERVENTION AND SPD REMAINED AT 250 KTS. ABOUT 3 MI FROM ARNES THE FMC SPD BUG SUDDENLY SLOWED TO 180 KTS. I CALLED OUT SPD 180 AND RESELECTED 250 KTS. AS I WAS DOING THAT THE COPLT NOTICED THE ACFT SUDDENLY NOSE OVER AND START DSNDING AT 2000 FPM. HE CALLED OUT THAT WE WERE STILL 3 MI FROM ARNES AND SHOULD BE AT 10000 FT. MY ATTN WAS STILL FIXED ON THE AIRSPD BUG AND IT TOOK ME A FEW SECONDS TO REALIZE WHAT WAS HAPPENING. ABOUT THIS TIME; LAX APCH CTL CALLED AND ASKED US IF WE KNEW WE WERE DSNDING EARLY. I REACHED UP AND PRESSED; THE ALT HOLD BUTTON STOPPING THE DSCNT AT 9300 FT. ATC RECLRED US TO MAINTAIN 9000 FT TO ARNES AND THEN TOLD US TO CONTINUE THE APCH. I FEEL THE REASON FOR THIS ALT ERROR WAS A PROBLEM IN THE FMC CTLR AND ALSO BECAUSE WE HAD SELECTED 8000 FT IN THE ALT SELECTOR. IF WE HAD SELECTED EACH XING ALT IN THE PROFILE WE WOULD NOT HAVE DSNDED EARLY AND WOULD HAVE A BETTER PICTURE OF OUR POS ON THE PROFILE. INSTEAD OF HAVING TO DEAL WITH MULTIPLE PROBLEMS WE WOULD ONLY HAVE HAD A SPD PROBLEM TO DEAL WITH.