As far as I have seen, that usually happens on a 1.4 when the PCV valve in the manifold fails or when someone forgets the two places that form-a-gasket should be used on a replacement.How does the bottom of the #1 plug well look? Is there any oil down there? The 1.8's are prone to valve cover leaks which allow oil to seep into the well and short out the coil, which can damage it, as well as causing a misfire.
As I recall, the gen1 1.4T valve cover gaskets are prone to doing that, too. (Hopefully somebody with one will chime in here.)
When you swapped plugs and replaced the coil module, did you notice any oil on the #1 boot?
Doug
.