B767 CREW WAS ASSIGNED 6 SPD CHANGES IN ABOUT 4 MINS. THE ACFT AUTOMATION WAS UNABLE TO HANDLE IT AND AN ALT EXCURSION RESULTED.

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ACR XXX (CYYZ-LAX) DSNDING INTO LAX. WE HAD BEEN CLRED DIRECT CIVET AND THE CIVET 4 ARR. OUR SPD HAD BEEN 310 KTS. ATC REQUESTED 250 KTS. WE COMPLIED. WE WERE SWITCHED TO SOCAL APCH WHO ASSIGNED US 320 KTS. WE COMPLIED. ACFT WAS BEING FLOWN IN VNAV LNAV WITH SPD BRAKES EXTENDED TO MAINTAIN PROFILE. AFTER CIVET; THE ACFT LEVELED AND LEFT THE PROFILE. I SELECTED FLT LEVEL CHANGE IN AN ATTEMPT TO GET THE ACFT TO CONTINUE THE DSCNT. BECAUSE THE ACFT WAS BEING OPERATED IN VNAV; 8000 FT HAD BEEN SET IN THE ALT WINDOW OF THE AUTOPLT. HOWEVER; WHEN FLT LEVEL CHANGE WAS SELECTED; THE ALT WAS NOT RESELECTED TO THE 12000 FT RESTR (AT BREMR). AS SUCH; THE ACFT DSNDED BELOW THE 12000 FT LEVEL TO 11000 FT BEFORE I REALIZED MY MISTAKE. I DISCONNECTED AUTOPLT AND AUTOTHRUST; LEVELED THE ACFT AT 11000 FT AND BROUGHT THE MISTAKE TO THE CAPT'S ATTN. HE ADVISED ME TO MAINTAIN THE ALT AND REACQUIRE THE VNAV PATH AND THEN CONTINUE THE DSCNT. ATC WAS NOT ADVISED NOR DID THEY QUESTION OR CALL ATTN TO OUR EXCURSION. I FEEL THAT CONTRIBUTING FACTORS TO THIS INCIDENT WERE POOR COM/COORD BTWN ATC SECTORS RESULTING IN MASSIVE SPD VARIATIONS. 310 KTS TO 250 KTS TO 320 KTS TO 250 KTS TO 210 KTS TO 170 KTS WITHIN A 20 MI SECTOR. THIS IS ALREADY A CRITICAL FMS ARR WITHOUT SPD CHANGE REQUESTS. SPD BRAKES ARE REQUIRED AS A NORMAL ROUTINE. IN THIS CASE IT WAS TOO MUCH FOR THE FMS AND REQUIRED US TO RELY ON A LESS AUTOMATED DSCNT (NON STOP) WHEREBY ERRORS COULD EASILY CREEP IN. OUR PERFORMANCE WAS DEGRADED (CREW COORD) BY A HEAVILY INCREASED WORKLOAD. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: PLT SAID HE HAS BEEN TO LAX 3 TIMES SINCE HE SUBMITTED HIS RPT. ON EACH OCCASION THEY HAVE HAD MULTIPLE STARS; RWY CHANGES; AND CHANGES TO THE POINT THAT THE ACFT FMC WAS UNABLE TO COPE. HE CONSIDERS THE LAX SIT THE MOST DIFFICULT OF ANY PLACE HE FLIES IN THE WORLD. HE FURTHER STATES THAT IT HAS GOTTEN MORE DIFFICULT THIS SUMMER. CREWS ON HIS ACR BEGIN BRIEFING ABOUT 45 MINS EARLIER THAN NORMAL AND TRY TO BRIEF FOR ALL POSSIBLE APCHS. THIS INCREASES COCKPIT WORKLOAD AND MAKES IT A PROB KEEPING THE DIFFERENT APCHS SEPARATED.