ACR FLT CREW LEVELED OFF AT INCORRECT ALT. SET UP POTENTIAL CONFLICT. REPORTER STATES ATC FAC HAS MORE TRAFFIC THAN IT CAN SAFELY HANDLE.

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THIS INCIDENT BEGAN JUST PRIOR TO DSCNT INTO LAX AREA ON A NONSTOP LFT FROM BOS. WE WERE CLRED PGS J-128 TO LAX AND SHORTLY AFTER PASSING OVER THE COLORADO RIVER WERE CLRED TO DESCENDED TO FL240 WITH PLT'S DISCRETION OUT OF FL310. DSCNT WAS BEGUN AND THEN STOPPED AT FL310. SHORTLY THEREAFTER THE CTLR CLRED US TO FL220. SINCE WE HAD PREVIOUSLY BEEN CLRED PLT'S DISCRETION AND SINCE THIS WAS AN UNUSUALLY LOW ALT FOR OUR POS; I ASKED THE CTLR FOR CONFIRMATION THAT THE CLRNC WAS ALSO PLT'S DISCRETION. THE CTLR'S REPLY WAS RUSHED AND UNINTELLIGIBLE; SO I ASKED A SECOND TIME IF CLRNC WAS PLT'S DISCRETION TO MAKE CERTAIN THERE WAS NO MISUNDERSTANDING. HIS REPLY WAS SIMPLY; 'YES.' DISCUSSION WITH THE OTHER 2 CREW MEMBERS IN THE COCKPIT CONFIRMED AN AGREEMENT ON WHAT EACH OF US HEARD THE CTLR SAY AND THAT WE WERE IN FACT CLRED TO FL220 PLT'S DISCRETION. WE REMAINED AT FL310. 2 OR 3 MINS LATER THE CTLR CALLED US; HIS VOICE TINGED WITH PANIC; SAYING: 'YOU ARE STILL AT 310; I CLRED YOU 10 MI BACK TO DESCENDED TO 220; DESCENDED IMMEDIATELY TO 220 AND SLOW TO 250!!!' I ACKNOWLEDGED THE CLRNC AND BEGAN DSCNT FROM FL310. IT IS COMMON IN THIS AREA TO BE VECTORED; SLOWED; ASKED TO SPD UP; ETC. THE VOL OF TFC INTO THE LAX APCH SYS IS APPARENTLY JUST TOO MUCH FOR THE SYS TO HANDLE ON THE HDOF FROM THE CENTER TO APCH CTL IN LAX. THE CTLR IN THIS TIME FRAME WAS VERY BUSY AND THE TONE OF HIS VOICE INDICATED HE WAS OVERLOADED AND BARELY ABLE TO HANDLE THE VOL OF TFC HE HAD. COM; WHEN EITHER THE CTLR OR PLT IS UNDER UNUSUAL STRESS; TENDS TO LOSE ITS FORMALITY AND THEREFORE AS IN THIS INSTANCE; CLARITY IS LOST. ALTHOUGH I ASKED TWICE FOR A CLARIFICATION OF THE CLRNC AND ALL 3 COCKPIT CREWMEMBERS UNDERSTOOD THAT WE HAD IT; I SHOULD HAVE INSISTED ON A FULL CLRNC READBACK FROM THE CTLR. I BELIEVE THE ROOT CAUSE OF THE INCIDENT IS THE OVERLOADED HDOF SYS INTO LAX COUPLED WITH THE GIVING AND ACCEPTING OF LESS THAN FORMAL WORKING IN A CLRNC DURING A STRESSFUL TIME. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING: RPTR SAYS THAT ONE CENTER CTLR GAVE THE FIRST DSCNT CLRNC WITH PLT'S DISCRETION OUT OF FL310; AND THEN THEY WERE CHANGED TO ANOTHER CENTER CTLR FREQ. THIS SECOND ZLA CTLR MUST HAVE NOTED CONFLICTING TFC AT FL310 AND TRIED TO GET RPTR TO DESCENDED TO FL220. RPTR QUESTIONED CTLR BECAUSE THEY WERE TOO FAR OUT TO GO THAT LOW. ALL FLT CREW MEMBERS THOUGHT CTLR SAID OK FOR PLT'S DISCRETION DSCNT. CTLR WAS EXTREMELY BUSY AND THEY THOUGHT HE HAD MORE TFC THAN HE COULD HANDLE. REALIZED LATER THAT CTLR WANTED THEM TO DESCENDED IMMEDIATELY. I COUNSELED RPTR THAT HE HAD LEVELLED OFF IMPROPERLY AT FL310 WHEN HE HAD BEEN TOLD; 'PLT DISCRETION OUT OF FL310.' EXPLAINED TO HIM THAT HE SHOULD HAVE CONTINUED DSCNT TO NEXT LOWER ALT (FL290) BEFORE APPLYING PLT'S DISCRETION.