Silver Wings

My Sti2 'stutters' just as I touch the throttle from idle and when 'blipping' the throttle on down changes (ie heel and toeing it) it seems to hesitate so I don't get the full benefit of the 'blip'. I'm not sure if it perhaps started this after fitting a Magnex exhaust or perhaps after fitting new Denso (non-iridium) plugs. The engine runs very well otherwise. Anyone any ideas? I'm sure the car didn't do this when I first got it a few months ago.

Colin