LOSS OF SEPARATION WHEN LGT DESCENDED AT A VERY RAPID RATE AFTER REPORTER HAD BEEN CLEARED TO 5000'.

Narrative

I WAS BEING VECTORED FOR AN LDA 18 APCH AT DCA. A RWY CHANGE WAS IN PROGRESS. FROM THE SOUND OF IT; THE CTLR WAS VERY BUSY. MY MODE C WAS RPTED INTERMITTENT AND AT TIMES INOP BY ZNY PRIOR TO THE HDOF TO DCA. AT THE TIME OF THE INCIDENT I WAS GIVEN A CLRNC FROM 6000-5000'. I BEGAN A 500 FPM DSCNT. DUE TO THE RWY CHARGE I WAS REVIEWING THE NEW APCH PLATE AND MY F/O HAD THE WATCH BRIEFLY; WHICH IS MY COMPANY PROC. OUT OF APPROX 5600' HE SAW ANOTHER ACR LGT CONVERGING FROM 1-2 O'CLOCK POS AT WHAT APPEARED TO BE A COLLISION COURSE. HE TOOK IMMEDIATE EVASIVE ACTION AND INFORMED ME AT THE SAME TIME OF WHY AND WHAT HE WAS DOING. I THEN TOOK CTL OF MY ACFT AND DIDN'T CHANGE A THING HE HAD DONE. THE OTHER FLT THEN ASKED APCH WHAT MY ALT WAS. THE CTLR RESPONDED; 'HE'S LEVEL AT 5000'.' I IMMEDIATELY RESPONDED 'I AM NOT LEVEL AT 5000'; I AM NOW OUT OF 5400' DSNDING TO 5000' AND THAT WAS VERY CLOSE.' AT THIS POINT THE CTLR WAS OBVIOUSLY SHAKEN AND ON AT LEAST 2 OCCASIONS CALLED US BY THE WRONG FLT #. MY FEELINGS ARE THAT THE CTLR WAS TOO BUSY; HE MADE AN ASSUMPTION (MY ALT) THAT HE SHOULDN'T HAVE; THE OTHER FLT SHOULDN'T HAVE BEEN THAT CLOSE TO ME OR MY FLT PATH IN THE FIRST PLACE. HE HAD BEEN GIVEN A DSCNT FROM 9000-6000' AT APPROX THE SAME TIME I WAS GIVEN A DSCNT FROM 6000-5000'; HIS DSCNT OBVIOUSLY MUCH FASTER THAN MINE. I ONLY HAD 1000' TO LOSE. I'VE BEEN AN AIRLINE PLT FOR OVER 20 YRS AND I BELIEVE THE ATC SYS IN MANY AREAS IN OUR COUNTRY IS STRETCHED TO THE LIMIT OF ACCEPTABLE SAFETY.