Just a thought-- I had the same problem on my ZETEC. Started missing under load, when I was pulling away from a stop or accelerating. I had changed the plugs and wires about 3 months prior for the same problem and had cured it. I pulled the new plugs and looked at the tips and they were fine, however the porcelain part had a problem. The two plugs on the shortest wires had carbon tracks down the porcelain. If you haven't seen it, a carbon track looks like a pencil line drawn on the porcelain. I scraped the carbon track off with a pocket knife and put some di-electric waterproofing compound on the plugs. Then I gooped some compound into the wire boot before putting it back on the plug so it would goosh around the plug. Been running great for two months now. Carbon tracks happen when there is electrical arcing around the outside of the plug. I'm guessin' these coil packs packs shoot a pretty hot spark and the shorter spark plug wires get it to the plug better.
