POTENTIAL CONFLICT ENSUES AS ALTDEV ALT OVERSHOT IN DSCNT.

Narrative

COPLT WAS FLYING THE AIRPLANE. WE RECEIVED CLRNC TO CROSS THE BRDGE INTXN AT 11000 FT. THE DSCNT WAS STARTED FROM FL220 AT 20-25 MI FROM BRDGE INTXN. PASSING THROUGH 15000 FT; ZMA ADVISED US OF TFC AHEAD OF 10000 FT THAT WE WOULD BE OVERTAKING. PASSING THROUGH 13000 FT; CTR GAVE US A FREQ CHANGE TO TPA APCH. AT THAT TIME WE WERE 1 MI BEHIND THE TFC (WITH VISUAL CONTACT OF THE ACFT) AND DSNDING AT 4000 FPM. AS I WAS CHANGING FREQ TO APCH CTL; THE COPLT STARTED TO LEVEL OFF AT 11000 FT; BUT WAS SLOW IN DOING SO. I WAS TALKING TO APCH AND POINTING TO THE ALTIMETER FOR THE COPLT TO SEE THAT HE NEEDED TO LEVEL OFF. THE DSCNT WAS CONTINUED DOWN TO 10600 FT BEFORE IT WAS ARRESTED AND RETURNED TO 11000 FT. THE ACFT BELOW US (A COMMUTER FLT WITH TCASII) QUESTIONED APCH IN A HIGH PITCHED VOICE AS TO WHAT WE WERE DOING. WE HAD SET OFF AN ALERT ON THE TCASII. THEY NEVER SAID IF THEY HAD RESPONDED TO THE ALERT WITH AN ALT CHANGE APCH TOLD THEM THAT HE SHOWED US AT 11000 FT AND THAT THAT WAS THE ALT WE WERE CLRED TO. APPARENTLY THE APCH CTLR DID NOT GET AN ALERT ON HIS EQUIP. IN THE PAST; I HAVE SET OFF TCASII ALERTS WITH HIGH RATES OF DSCNT; BUT NEVER BEFORE HAD I BROKEN A 1000 FT ALT SEPARATION. HIGH RATES OF DSCNT ARE COMMON WITH OUR FLYING. WE ARE USED TO IT AND MOST CTLRS KNOW THAT WE MAKE STEEP DSCNTS AND STILL MAKE RESTRS. THE COPLT NORMALLY DOES A GOOD JOB; THIS TIME HE SCREWED UP. I WAS DISTRACTED BY THE FREQ CHANGE. WHEN I DID LOOK AT HIS DSCNT; IT DID NOT LOOK OUT OF THE ORDINARY. FROM NOW ON; I WILL DELAY THE RADIO CALL TO INSURE THAT THE PROPER ALTS ARE ACHIEVED!