I have a 2006 5.4 screw XLT and about a month or so ago it started stuttering around 40-50 mph. It seemed to only happen when I had slowed down during long turns and then gave it gas coming out of the turn or highway driving after braking and then accelerating to higher speeds. The previous owner had ford change the plugs and it's at that 100k mark since then. At first I thought it was an issue with the trans but after a few weeks it got a little worse and resembled a coil misfire but wasn't throwing codes and after 50mph it would fire flawlessly. At any rate, since ford had performed the anti-seize TSP and I was due for a plug change anyway, I decided to go ahead and replace the plugs with the SP515s and grabbed new coils as well. I had zero issues with the plugs, there were no breaks and the truck drives like a caddy now. Just wanted to thank some of you who posted about similar issues and directed me towards a plug change.

 

Congrats on your diagnosis and fix. Thanks for posting your experience.