There are certain Spark plugs which have a too small ceramic diameter causing the plug lead to short down the side of the plug to the head.
I think ford used to put some special (non conducting) jelly in the plug leads to stop this.
I had a 105 zetec - used to missfire at random points for the three years i owned it - no logic to it - got pissed off - sold it then spoke to a ford mechanic and that is what he told me !!

You need to buy some specific make plugs - sorry can't remember the make that work. I would think that Motocraft ones would be ok - from ford !!