CAPT OF A B757 OVERSHOT CLB ALT DURING DEP DUE TO DISTR OF HDG CHANGE. RPTR IMMEDIATELY CORRECTED BACK; BUT SLOWLY FOR PAX COMFORT.

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SHORTLY AFTER TKOF FROM TORONTO ON PEARSON 6 DEP FROM RWY 24R; WE WERE CLRED TO 7000 FT. THE DEP CALLS FOR A TURN TO 245 DEGS AT THE 3.9 DME FIX FROM THE YTP VOR. ON CLBOUT; JUST AFTER TURNING TO THE 245 DEG HDG; DEP INSTRUCTED US TO TURN L TO 220 DEGS. I FIXATED ON THE NEW HDG MOMENTARILY AND DIDN'T BEGIN MY LEVELOFF UNTIL NEARLY 7000 FT -- SIMULTANEOUSLY WITH THE FO'S CALL THAT I HAD PASSED 7000 FT! I HAD BASICALLY FIXATED ON THE HDG WHILE HAND FLYING AN EXTREMELY LIGHT B757 THAT WAS CLBING AT CLOSE TO 6000 FPM. THEN I EXACERBATED THE SIT BY CHOOSING THE LEVEL OFF AND RETURN TO 7000 FT AS GENTLY AS POSSIBLE. WHILE I MAY HAVE SUCCEEDED IN SPARING SOME PAX THE DISCOMFORT OF AN ABRUPT LEVELOFF; THE GENTLE MANEUVER I USED TOOK US UP TO 7400 FT; WELL OUTSIDE TO 300 FT OF LEEWAY WE MAY HAVE BEFORE ALARMS START GOING OFF. AT ABOUT THE TIME I BEGAN MY TARDY LEVELOFF AND DSCNT BACK TO 7000 FT; WE ALSO HAD A TCASII TA INDICATING WE WERE STARTING TO GET CLOSE TO ANOTHER AIRPLANE. ATC CONFIRMED THIS; ADVISING US OF THE TFC ON DSCNT INTO TORONTO. WE DID NOT CLOSE ON THIS TFC ENOUGH TO GENERATE AN RA -- WHEW!