Normally the coilpack bracket has a supressor bolted to the side, a grey block that is held with one screw.
If it has no provision for one on the bracket, then it may be it is elsewhere.
It could also be a lead breaking down, is there any misfiring at all especially when cold or damp ?, is the wire between the body and the bonnet in place, as this stops the bonnet causing trouble acting as a aerial, I assume your aerial is body mounted ?
Are you using resistor plugs, you should be anyway, just steer clear of bosch plugs on these.
tabetha