1)are the plugs tightened up enough, common problem 2)are the leads fully connected on the plugs, not as easy as it sounds without the proper tool 3)faulty leads, try the old ones again 4)coil pack damaged/cracked , if it is you can coat it in epoxy resin to re-insulate it which will sometimes solve the problem and save a few hundered quid 5) leads put back on in correct order? If its suddenly playing up after new leads, the chances are it'll be something small i would think happy hunting! haha