I had a similar problem on my 4/4 with a 1.8 black top zetec. I changed the coil pack, leads and plugs. An experienced friend (garage owner) warned against changing just one item at a time. I purchased ford plug leads having been warned against using poor quality leads. I used a Bosch coil pack and NGK plugs. The problem was resolved. Don't know which was the faulty item but I suspect it was the coil pack.
