engine issues

225 slant six, 72 dart, ok when the engine is cold it runs, accelerates and idles and everything is fine, BUT after the engine warms up its not ok, i can give it a little throttle and its fine all pistons fire and the car accelerates but there is a "sweet spot" on the throttle so that after i pass that point it starts misfiring like its not getting enough gas or something, bottom line is i cant for the life of me begin to figure out what this could be! there is a gas can extension tube that fell into the gas tank but that would not explain why it only happens before the engine warms up. i checked the choke and it opens like it should and seems to be fine. any way, if i could get someone to narrow the problem down a little i might be able to fix it or figure this one out.