ZDC CTLR EXPERIENCED LOSS OF SEPARATION WHEN A P180 ASCENDED FROM ASSIGNED FL370 AND CONFLICTED WITH TFC AT FL380.

Narrative

I WAS WORKING SECTOR 38 (TAR RIVER SECTOR) AT ZDC; WITH A D-SIDE RADAR ASSISTANT. THERE WAS A HIGH VOLUME OF TFC; 20+ ACFT AND A HIGH LEVEL OF COMPLEXITY. MORE THAN 1/2 THE ACFT WERE TRANSITIONING -- CLBING OR DSNDING. 5 OR 6 ACFT WERE ON RADAR VECTORS FOR IN-TRAIL SPACING INTO DC METRO AREA -- IAD; DCA; BWI. ANOTHER PAIR OF NY BOUND ACFT WERE ON VECTORS TO MAINTAIN 5 NM FROM EACH OTHER. A STUCK MIKE OCCURRED FOR A 2 MIN PERIOD. THIS PUT US 'VERY' BEHIND IN THE SITUATION AND WORKLOAD INCREASED DRAMATICALLY. ACFT X; P180; REQUIRED A TOTAL RERTE. ONCE THE WORKLOAD PERMITTED THE RERTE WAS ISSUED ALONG WITH A DSCNT CLRNC TO FL360. WE DID NOT NOTICE DUE TO OTHER PRIORITIES; BUT ACFT X CLBED UP TO FL378 BEFORE BEGINNING THE DSCNT TO FL360. SEVERAL MINS LATER; ACR Y; LEVEL AT FL380; QUESTIONED US ABOUT 'TFC OFF HIS L.' I ASSUMED IT WAS A BAD MODE C. ACR Y WAS NOT HAPPY WITH THAT EXPLANATION AND WANTED THE NUMBER TO CALL. AN INVESTIGATION WAS CONDUCTED AND IT WAS DISCOVERED THAT THE P180 INDEED DID CLB. WE CHKED OUR FLT PROGRESS STRIP AND I QUESTIONED ACFT X AND BOTH INDICATED BEING AT FL370. SUPPLEMENTAL INFO FROM ACN 684210: WHILE ENRTE TO MMU; ZDC HAD US LEVEL AT FL370; A DSND AND REROUTE CLRNC TO FL350 WAS ASSIGNED. I PROCEEDED TO SELECT THE AUTOPLT TO A DSCNT AND LOOKED AWAY TO SET THE NEW RTE CLRNC. APPROX 30 SECONDS PASSED AND I CHKED MY INST SO AT THAT TIME I NOTICED THAT THE ACFT WAS CLBING THROUGH FL375. I DISCONNECTED THE AUTOPLT AND INITIATED A DSCNT TO FL350. I BELIEVE THE CAUSE OF THIS INCIDENT WAS FAILURE OF MONITORING THE AUTOPLT.