ZOA CTLR DESCRIBED CONFLICT EVENT BTWN FL190 AND FL210 WHEN CLB RATES WERE MISJUDGED.

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2 PAX ACFT IN CLB PHASE WERE NBOUND AND THE B767 WAS ALSO IN CLB PHASE WBOUND. THE NBOUND ACFT WERE CLING AT APPROX 1500 FPM WHILE THE B767 WAS CLBING ABOUT 1000 FPM. THE B767 WAS BELOW THE XING TFC AND NOT CLBING AS QUICKLY WHEN I ISSUED A CLB TO THE SAME ALT AS THE OTHERS. ALL ACFT WOULD CONTINUE TO CLB IN THE NEXT HIGHER STRATUM. I EXPECTED TO GET A HIGHER ALT FROM THE NEXT CTLR FOR THE NBOUND ACFT IN CONFLICT SO THAT IT WOULD NOT LEVELOFF FOR SOME REASON AND BECOME A PROB. JUST BEFORE INITIATING THE LAND LINE CALL I OBSERVED THE ALT OF THE B767 GO TO 'XXX.' I THOUGHT THIS WAS DUE TO THE PROX OF THE ACFT LATERALLY FROM THE RADAR ANTENNA. I QUESTIONED THE ALT OF THE B767 AND IT RPTED FL182 AND THE OTHER ACFT WAS LEAVING FL195. I REALIZED THE B767 HAD INCREASED ITS RATE OF CLB SUBSTANTIALLY AND TO THE POINT THAT THE RADAR DISPLAYED 'XXX' SINCE THE ACFT WAS OUTSIDE THE COMPUTER'S PROGRAMMED CLB ENVELOPE. I ORDERED THE B767 TO 'LEVELOFF' AND TOLD THE MD90 TO EXPEDITE CLB THROUGH FL210 FOR TFC. THE B767 RPTED THE OTHER ACFT 'IN SIGHT; NO FACTOR.' I OBSERVED THE MD90 LEAVING FL204 AND ASKED THE B757 TO RPT LEVEL AT FL190. THE B767 RPTED LEVELING AT FL196. I DO NOT KNOW IF SEPARATION WAS MAINTAINED. THE POTENTIAL CONFLICT ALERT FUNCTION ACTIVATED ON MY DISPLAY AFTER I HAD INITIATED ACTION AND APPARENTLY DID NOT RPT A LOSS OF SEPARATION TO THE WATCH DESK. THE PROB WAS OF MY OWN MAKING BY ANTICIPATING BASED ON CURRENT CLB RATES. MAJOR CONTRIBUTING FACTOR WAS THAT THE ALT DISPLAYED 'XXX.' THE COMPUTER SHOULD DISPLAY MODE C DATA OVER A WIDER ENVELOPE. I WAS ALSO WORKING ALONE AT THE SECTOR WHILE TRAINING EXISTED AT 3 OF THE 6 SECTORS IN MY AREA. I WAS WORKING AN OVERTIME SHIFT ON A DAY FOLLOWING A MID SHIFT AND WAS FATIGUED.