Out of curiosity, my friend has a 01 F150 and same thing. It was cylinder 4 the last time and when she got the truck checked this time they said it was cylinder 7. These COP are a pain in the ****. You get them wet and they run like crap.
Just my 0.2 worth.

 

Yeah the COP's are a pain. It cured my problem though. #'s 5-8 are the worst in my opinion.

 

Unfortunately the way these engines are designed the plug wells love to hold water, especially when they are washed, of coarse the coils die quick after the engine is washed but, the computer has to read a miss on at least four to five warm up cycles before it will set a code. They are easy to diagnose once the light comes on. I have done enough COP changes that none of them are all that hard anymore. It is just a matter of knowing what to move and having the right sockets and wobblies and extensions. Try changing them on the expies, with rear heat and air, they have twice the heater hoses on the passenger side to work around.