Coils can break down in the internal windings, especially if they are run open circuit, not fitted to the plug. Most of the shorting takes place at the rubber boot that covers the plug. They age over time to conduct down the side, shorting the spark from the plug. If you check the plug covers carefully, you may see a black line running up the side of it. This can be scratched out, and sprayed with a light oil to seal it, but they can be replaced with some off a set of 'after market' plug leads.
The leads that run to the coils, should not cause a misfire, but I would seal them anyway.
Harvey.
