Kinda late on this one, but....
Did you polish the intake ports? Intake ports shouldn't be polished, (that's what they say nowadays) the injected fuel will, in many cases, "wet out": liquid gasoline doesn't burn. At higher rpm's, it's not such a problem, there's enough airflow to rip apart drops of gasoline, but at lower rpm's a matte or slightly rough finish to the ports will give liquid gasoline enough traction to be picked apart by the lower airspeeds. It may cost a whole horsepower or so in an allout tuning effort, but it's becoming common practice simply due to the driveability concerns. But that's just a possibility! Check the easy stuff first, vacuum leaks and so forth.