AN ARTCC RADAR CTLR RPT ON A POTENTIAL CONFLICT AND A SYS ERROR WHEN HE LOST SEPARATION BTWN 2 ACR FLTS NEAR SLK; NY.

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ON DEC/XA/99 AT APPROX PM43; I WAS WORKING THE RADAR POS AT SECTOR 52/53 MPV/PLB SECTOR. ACFT X; AN MD80/R WAS SW OF SLK VOR PROCEEDING TO YUL VIA PLB PLB2 ARR AT FL330. ACR Y WAS SE OF SLK VOR PROCEEDING SW VIA SYR VOR AT FL310. A THIRD ACFT Z WAS AT FL370 S OF PLB VOR INBOUND TO YUL. BOTH ACFT Z AND ACR X WOULD ARRIVE AT PLB VOR AT THE SAME TIME; SO IN ORDER TO PROVIDE SPACING TO YUL; I ISSUED ACR X A CLRNC DIRECT YUL. THIS TURN OF AT LEAST 30 DEGS L OF HIS PRESENT COURSE WOULD TAKE HIM AWAY FROM THE FLT PATH OF ACR Y. APPROX 12 SECONDS LATER; I ISSUED ACR X A DSCNT CLRNC TO 11000 FT. I EXPECTED THAT ACR X WOULD START HIS TURN AS INSTRUCTED AND THEN BEGIN HIS DSCNT. I EXPECTED THAT THE TURN WOULD PROVIDE THE REQUIRED SEPARATION. I THEN MOVED TO THE D SIDE TO CHK THE STRIPS SINCE THERE WAS NO D SIDE (NON RADAR POS) AVAILABLE DUE TO STAFFING. WHEN I RECHKED THE RADAR I OBSERVED THE CONFLICT AND THE CONFLICT ALERT ACTIVATED. ACR X DATA BLOCK INDICATED AN ALT OF FL328. I TOOK IMMEDIATE ACTION TO RESOLVE THE CONFLICT BY ISSUING A CLRNC TO ACR X TO CLB AND MAINTAIN FL330.