I've heard of this problem on the 5.4 engines in general. It happened to a guy who was argueing with the Service Manager when I was at the dealership getting an oil change. Anyway from what I overheard, he (Ford service mgr.)admits this is an issue, but said that this was probably due to an improperly installed plug, which has been the findings of others they had seen.

Sorry, but that's all I have on the subject.

Good Luck!