i have had a similar problem with my Fiat Punto, and it was the ht leads, is due to the leads being made out of silicone now and they crack easily!
This has happened twice on my fiat in just over 20k miles so i would personally check the leads one at a time, disconect one and see if effects the tick over of the engine if not then its that lead, but if you are going to replace one lead you mays well do the lot