You need another mechanic. Any mechanic with any experience at all would have changed the plugs along with the coil packs and boots on a 100K mile vehicle with misfire issues, even if they were the plugs known to have breaking issues. Had to have them changed on my 05, and they only broke three plugs. It takes a little time to get them out without breaking all of them and costs more. I had a $600 bill and no coil packs were changed, just the plugs and boots. My mechanic told me those were the most troublesome due to arcing (as noted earlier) and recommended not changing the coils unless needed -- they were easy to get to anyway (compared to plugs), and he wouldn't change the plugs without changing boots. Still have the original coils with no issues at 106K now (only put 3-4K a year on it -- mainly a camper tow vehicle I drive once a month even if not taking camper anywhere).
I'm a reasonably knowledgeable mechanic myself, though mostly for older cars (up to about early 90s), and not that familiar with the 5.4L -- just read enough to know the plugs and boots are a bigger issues, and many coil packs are replaced when the issue was the plugs and boots simply because the plugs are hard to get to and earlier ones prone to breaking. I'd intended to change the plugs, but a good look under the hood revealed that only three were relatively easy to get to, another two without too much difficulty, the other three required moving a lot of electrical components. Closed hood and took it to someone more familiar with them. I could have done it, but it would have been an all day job for me, figuring what to move where as I went. Wasn't worth saving a few hundred knowing what aggravation it would be for a first timer. Let someone who had done a few do it...
