MLG FAILED TO ADHERE TO AMENDED ALT AND HAD LESS THAN STANDARD SEPARATION WITH PREVIOUSLY ISSUED SMT TRAFFIC.

Narrative

FLT FAT TO RNO WAS GIVEN CLRNC TO CLB TO MAINTAIN 10000' BY CLRNC DELIVERY. UPON DEP AND IN A LEFT TURN PASSING 3600'; DEP CTL MODIFIED CLRNC TO MAINTAIN 4000'; TFC AT 5000'; 3 MI AT 4 O'CLOCK (AN SMT). OUR RATE OF CLB AT TIME OF CLRNC TO MAINTAIN 4000' WAS 6000 FPM IN AN MLG X. IT WAS PHYSICALLY IMPOSSIBLE AND UNSAFE TO LEVEL OFF THAT FAST W/O POSSIBLY PHYSICALLY HURTING PAX AND F/A'S. AS BEST MY F/O COULD; HE LEVELED OUT 4250' MAX AND IMMEDIATE DSCNT BACK TO CLRED ALT OF 4000'. RATE OF CLB WAS TOO EXCESSIVE TO SAFELY STOP OUT RATE OF CLB TO LEVEL OFF THAT FAST. UPON REFLECTION ON INCIDENT; IT APPEARS THAT WITH INHERENT DISCREPANCIES OF ATC RADARS TO KEEP UP WITH RATES OF CLB AND DSCNTS ON SCOPE READOUTS; ESPECIALLY WHEN RATES OF CLB ARE EXCESSIVE (IE; MLG X WITH VERY LIGHT WTS); THAT OUR ALT AT TIME OF CLRNC AND DEP SCOPE MAY HAVE BEEN ONLY 3000' (APPARENTLY CAPABLE OF LEVELING OFF IN 1000'). NOT SO--WE WERE ALREADY AT 4600'. WE DISCUSSED THIS OVER THE RADIO WITH DEP CTL EXPLAINING OUR RATE OF CLB PROB AND LEVELING OFF.