CTLR MISJUDGED A320 CLB RATE AT HIGH ALT RESULTING IN LOSS OF REQUIRED SEPARATION.

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THE A320 DEPARTED DEN ON THE WAY TO PHX. THE A320 CHKED ON FREQ FROM THE LOW ALT SECTOR CLBING TO FL260. I CLRED THE A320 TO FL390; REQUESTED ALT. A COUPLE OF MINS LATER; XING TFC WAS OBSERVED FROM THE W AT FL370. I ASKED THE A320 PLT IF THEY COULD REACH FL390 IN 4 MINS. THE PLT STATED IT WOULD BE CLOSE. I FIGURED THE A320 HAD 6 MINS BEFORE SEPARATION WOULD BE LOST. FROM OBSERVING THE MODE C OF THE A320; THE A320 WAS CLBING AT AN 1800-2000 FPM RATE THROUGH FL370. FROM FL370 TO FL380 THE A320 CLB RATE WAS ABOUT 1000 FPM. THE A320 CLB RATE REDUCED TO 500 FPM TO FL385. THE A320 THEN LEVELED OFF AT FL385 FOR APPROX 2 MINS. THIS IS WHEN SEPARATION WAS LOST. THE A320 TOOK ABOUT ANOTHER MIN OR 2 TO REACH FL390. THE VERY GOOD CLB RATE THROUGH FL380 AND ADEQUATE CLB RATE TO FL385 GAVE ME A FALSE SENSE OF SECURITY. I FELT THE A320 WOULD REACH FL390 BEFORE SEPARATION WOULD BE LOST. THEREFORE; I TOOK NO OTHER ACTIONS. THE FACT THAT THE A320 CLBS WELL EXCEPT FOR THE FINAL FEW HUNDRED FT TO FL390 IS ONE I WILL DEFINITELY REMEMBER.