SMT Y DSCNT TO OCCUPIED ALT HAD LTSS FROM ATX X. SYS ERROR.

Narrative

ATX X WAS INBOUND FROM THE S ON ALTTA ARR TO FAT AT 14000 FT. SMT Y WAS ON THE SAME RTE AT 15000 FT AND 1 MI IN TRAIL WITH A SLIGHT OVERTAKE. A FLT OF 2 FGTS WERE ALSO ON A RECOVERY TO FAT FROM OVER NLC. THE FGT'S FLT WAS VECTORED EBOUND; DSNDED AND SPLIT INTO 2 SINGLE ACFT. ATX X WAS ISSUED A DSCNT CLRNC (NO REPLY) AND SLOWED TO FOLLOW THE FGTS. THE FGTS WERE SEQUENCED IN FRONT OF BOTH ATX X AND SMT Y. THE ALT READOUT ON ATX X WAS LOST (INTERMITTENT) FOR A PERIOD OF OVER 35 SECONDS. THE 'DATA' BLOCKS OF THE 4 ACFT WERE CLOSE; AND THE CTLR THOUGHT HE OBSERVED AN ALT OF 12700 FT IN ATX X DATA BLOCK. AT THIS TIME SMT Y WAS SLIGHTLY AHEAD OF ATX X. SMT Y WAS DSNDED TO FL140. CONFLICT ALERT ACTIVATED SHORTLY AFTERWARDS AND SHOWED ATX X AT 13900 FT AND SMT Y AT 14400 FT. AGAIN ATX X WAS TOLD TO DSND TO 11000 FT BUT NOT IN TIME TO PREVENT LOSS OF SEPARATION. 2 MINS PRIOR TO ERROR; SECTOR WAS SPLIT (D-SIDE ADDED). THIS AND NUMEROUS UHF ACFT ON FREQ ADDED TO DISTR ON PART OF CTLR TO MONITOR MODE C ON ATX X.