A B767 FLC FAILS TO FOLLOW THE DSCNT PROFILE AS ISSUED BY AN OVERWORKED APCH CTLR AT ATL; GA.

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UPON CONTACTING APCH CTL; WE IMMEDIATELY DISCERNED THAT THE CTLR WAS OVERLOADED. HE DID NOT ANSWER ACFT CALLING HIM; AND INSTEAD ISSUED RAPID-FIRE CLRNCS TO SUCCESSIVE ACFT WITHOUT GIVING THEM TIME TO ACKNOWLEDGE. HE ISSUED A CLRNC; APPARENTLY TO US USING THE WRONG CALL SIGN. WHEN QUESTIONED ABOUT THE CALL SIGN HE DID NOT ACKNOWLEDGE THE ERROR; BUT MERELY REISSUED THE CLRNC. THESE ARE SIGNS OF 1 CTLR TRYING TO DO THE WORK OF 2. INDEED; ATL WAS NOT EVEN BUSY AT THAT TIME OF MORNING. AFTER GOING TO THE NEXT FREQ WE GOT THE SAME KIND OF RAPID-FIRE CLRNCS. UPON LISTENING TO THE CTLR FOR A SHORT PERIOD; IT WAS DETERMINED THAT HE WAS WORKING THE N AND S APCH CORRIDORS SIMULTANEOUSLY. WE HAD BEEN DSNDING TO 5000 FT WHEN THE CTLR GAVE US A 360 DEG HDG AND DSCNT TO 3500 FT. I TURNED THE ALT CTL TO 3500 FT AND THE HDG SELECT TO 360 DEGS. THE DSCNT MODE WAS IN FLT LEVEL CHANGE; AND I HAD EXPECTED IT TO CONTINUE DSNDING US TO 3500 FT. SIMULTANEOUSLY; I BECAME VERY CONCERNED ABOUT THE ACFT ON THE OTHER SIDE OF THE ARPT. WE GOT A TCASII ADVISORY AT THE SAME TIME THAT A SECOND VOICE (THE FINAL MONITOR) ISSUED US A 060 DEG HDG AND A VISUAL APCH CLRNC. I LOOKED AT OUR ALT AND WAS SHOCKED TO SEE THAT WE WERE NOT DSNDING. THE DSCNT MODE HAD OBVIOUSLY GONE TO ALT CAPTURE BEFORE I HAD SELECTED 3500 FT EVEN THOUGH THE ACFT HAD NOT YET REACHED 5000 FT. THE COPLT FAILED TO NOTICE THIS TOO. WE IMMEDIATELY DSNDED FROM 5000 FT AND NO RA WAS GIVEN BY TCASII. I BELIEVE THAT THE CTLR FAILED TO NOTICE OUR ALT WAS STILL 5000 FT AS WE APCHED THE N CORRIDOR BECAUSE HE WAS TOO BUSY CTLING BOTH N AND S APCHS. THUS; THE THIRD BACKUP MISSED. THE FINAL MONITOR SAW THE ERROR; BUT NOT IN TIME FOR A TCASII ALERT TO BE AVERTED. I THINK THAT WHEN CTLRS ARE WORKING 2 DIFFERENT SECTORS AND GIVEN AN ACFT A HDG CHANGE THAT HAS THE POTENTIAL FOR CREATING A CONFLICT; THAT THEY SHOULD GIVE THE ALT CHANGE FIRST AND THEN WAIT TO SEE IF THE ACFT RESPONDS BEFORE TURNING HI. I SHOULD NOT HAVE BEEN GIVEN A CLRNC TO TURN TO A 360 DEG HDG UNTIL IT WAS CLR THAT I WAS NO LONGER AT 5000 FT.