DURING A HIGH RATE OF DSCNT IN ORDER TO MAKE A XING RESTR; ATC AMENDED CLRNC TO STOP AT AN INTERMEDIATE ALT FOR XING VFR BUSINESS JET. THE STOPPING ALT WAS OVERSHOT DUE TO THE HIGH RATE OF DSCNT; AND THEN SUBSEQUENTLY OVERSHOT AGAIN WHEN ATTEMPTING TO GO BACK TO ASSIGNED ALT.

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WE WERE DSNDING INTO BOSTON ON THE GARDNER TWO ARR. BEFORE GARDNER WE HAD BEEN CLRED TO DSND TO 11000 FT FROM FL370. WE WERE TOLD TO KEEP OUR SPD UP TO PASS A LOWER ACFT. WE HAD DSNDED TO INTERMEDIATE ALTS AS THEY CLRED THE LOWER ACFT TO DSND. AFTER WE PASSED THE LOWER ACFT; WE WERE TOLD TO DSND TO 11000 FT; AT 11000 FT SLOW TO 250 KTS. WE WERE DOING 320 KTS AND DSNDING AT ABOUT 3000-3500 FPM. AT APPROX 17500 FT THEY TOLD US TO LEVEL AT 17000 FT. BY THE TIME WE STOPPED THE DSCNT WE WERE AT 16500 FT. WE IMMEDIATELY CLBED BACK TO 17000 FT. AT 16500 FT WE STARTED TO GET AN RA. IT WAS TELLING US TO DSND. WE HAD ALREADY STARTED OUR CLB AND THE ACFT AHEAD WAS 100 FT BELOW. WE CONTINUED OUR CLB AND WHEN WE LEVELED AT 17000 FT; THE TCASII SHOWED THE OPPOSITE ACFT 500 FT BELOW US. I SAW THE SMALL JET PASS TO OUR R SIDE; LESS THAN 1 MI. WE TOLD THE LADY CTLR SHE HAD NOT GIVEN US AMPLE WARNING TO MAINTAIN THE 17000 FT. A LOT WAS GOING ON: PASS LOWER ACFT; KEEP SPD UP; TRYING TO DSND TO CROSS BRONC INTXN AT 11000 FT. THEN; TRY TO STOP A STEEP DSCNT WITHIN 500 FT. I BELIEVE THE CTLR LOST THE AWARENESS OF THE VFR TFC IN CLOSE PROX TO US AND ALTS WE WERE AT.