A POTENTIAL CONFLICT OCCURS AS A DC9 OVERSHOT ITS ASSIGNED ALT OF 5000 FT WITH TFC 1 MI AT 6000 FT. ACFT WERE IN CLASS B OF IAH; TX.

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ATC RPTED TFC AT 11 O'CLOCK POS; 1 MI. WE WERE ASSIGNED 5000 FT MSL WITH TFC IN SIGHT. I NOTICED FO CLBING FAST AT 4900 FT; MADE CALL OF 'LEVEL HERE AT 5000 FT.' HE RESPONDED AND STARTED TO LEVEL. I CONTINUED TO WATCH TFC. ALT ALERT WENT OFF AT 5300 FT MSL; WENT TO 5400 FT BEFORE HE STARTED BACK DOWN TO 5000 FT. FO DISTR BY TFC. FO MORE CONCERNED WITH TFC THAN FLYING ACFT. SUPPLEMENTAL INFO FROM ACN 419272: GIVEN 5000 FT AS ALT LIMIT; 180 DEG HDG; NO SPD LIMIT. ENGAGED AUTOPLT AT 4500 FT TO LEVEL OFF WITH 340 KIAS. AT 4400 FT TA AND 4800 FT RA WAS ANNOUNCED. DISENGAGED AUTOPLT; TURNED TO HDG 200 DEGS WHILE GETTING A VISUAL ON THE TFC. WITH ACFT OUT OF TRIM AND 340 KIAS; A TFC ALERT; I WAS ONLY ABLE TO LEVEL OFF AT 5400 FT BEFORE RETURNING TO 5000 FT. THE ONLY REACTION FROM ATC WAS 'I ASSIGNED YOU 5000 FT.' THE DC9 ACFT IS NOT ABLE TO TRIM OUT LEVELOFF ALWAYS AT ALT TO WITHIN +/- 150 FT DUE TO TYPE OF SYS; AIRSPD AND RATE OF CLB. ATC NEVER GAVE CREW TA; BUT WAS ONLY CONCERNED OF US NOT BEING AT 5000 FT; AS THOUGH THE SIT OF TFC AVOIDANCE SNUCK UP ON HIM.