A320 FLC UNABLE TO COMPLY WITH ANTICIPATED LEVELOFF TIME.

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WE DEPARTED ATL FOR MSP. CLBING OUT; WE REQUESTED FL390 DUE TO RPTS OF TURB AT LOWER FLT LEVELS. FMGC SHOWED FL385 AS OPTIMUM ALT; FL391 MAX ALT (FL391 IS MAX CERTIFIED ALT FOR A320; NOT PERFORMANCE LIMIT). ZTL REQUESTED CLB THROUGH FL370 IN LESS THAN 8 MINS. THIS WAS COMPLIED WITH (IN APPROX 6 MINS). PASSING APPROX FL380; ZTL ASKED HOW LONG UNTIL WE COULD BE LEVEL AT FL390. WE RESPONDED 'ABOUT 2 MINS.' THIS WAS BASED ON OUR PRESENT RATE OF CLB. AT APPROX FL383 WE ENCOUNTERED LIGHT TURB AND A POSSIBLE LOSS OF HEADWIND. AIRSPD DECREASED TO 'GREEN DOT' (MINIMUM ZERO FLAP) AND THE ACFT STOPPED CLBING AT FL386. ACFT WOULD NOT CLB OR ACCELERATE; ALTHOUGH OUR FMGC WAS NOW SHOWING FL390 AS OPTIMUM ALT (GROSS WT 130000 LBS ON ECAM DISPLAY) AND MAX CONTINUOUS THRUST WAS OPERATING. ZTL ASKED US 3 TIMES TO 'RPT' OR 'CONFIRM' LEVEL AT FL390. EACH TIME WE RESPONDED; 'LEVEL FL386.' SENSING A TFC CONFLICT; WE OFFERED TO DSND AND WERE FINALLY GIVEN A VECTOR 30 DEGS TO THE R. WE WERE NOT NOTIFIED OF ANY CONFLICT; BUT AFTER BEING CLRED BACK ON COURSE WE WERE ADVISED TO CALL ZTL BY TELEPHONE UPON REACHING EST. I CALLED ZTL ON ARR AND WAS INFORMED BY ATC SHIFT SUPVR THAT A 'PLTDEV RPT' WAS BEING FILED. I SUSPECT THAT IN ADDITION TO ENCOUNTERING LIGHT TURB AND A LOSS OF HEADWIND; OUR POOR CLB PERFORMANCE MAY HAVE BEEN DUE TO A HIGHER ACFT GROSS WT THAN INDICATED. LOSS OF VERT SEPARATION WAS DUE TO CTLR'S FAILURE TO INITIATE OFF-COURSE VECTOR SOON ENOUGH. AT HIGH FLT LEVELS; OUR ESTIMATES OF 'TIME TO CLB' CAN BE VERY INACCURATE UNDER CERTAIN CIRCUMSTANCES.