Erik, I suspect the ferrules as being the area where water is leaking, entering the cylinder and seeping down the cylinder wall and out the exhaust. I ended up making new ferrules, specifically to suit my engine, to cure a very similar problem. If one of your ferrules unscrewed when a plug was removed, it was not really fitted properly.
Interesting that Roger recommends using silicone to seal the water dome. I finished up using cork gaskets as the most successful, the cork being about twice as thick as the club supplied ones. The cork was purchased from an Auto Supply store.
