07 5.4L Misfire Problem

[cmfish]

I recently bought a 2007 F150 with the 5.4L engine and 65k miles. I test drove it and everything was fine no problems. I paid the guy then as I was driving it back to my house it started to misfire when cruising above 40mph under light throttle. The CEL did not come on. I made a couple calls and a friend said it was likely a bad ignition coil. So I bought a coil and replaced it in each cylinder until it seemed like the problem was gone. I though this had solved the problem because I wasn't able to make it misfire. That was about a month ago, now recently it started misfiring again under the same conditions. I assumed that another coil went bad so I bought a whole set and replaced them all. I tested it out this morning and I'm having the same problem. It seems as though it gets worse sometimes and others it is hardly noticeable. It is only when cruising above 40 in top gear at low rpm and light throttle is applied. Any ideas on what may be causing this? Are there any computer updates or anything of that nature that could be causing this?

[LxMan1]

Might want to go ahead and change the plugs. May have a bad one.

[cmfish]

Thats what seems to be the consensus and was told today by the stealership that my plugs are bad. Mechanic said it was misfiring randomly on all the cylinders and slightly favored one side possibly indicating a bad cat on that side as well. For new plugs, cleaning injectors and updating computer gonna be roughly a grand....Then if the cats are bad thats another 1800 a piece, which are on backorder. And the best part is they are not responsible if they break a plug off when changing them out, I am responsible for the labor on removing the broken plug.

I have a hard time believing that all my plugs are bad with only 65k miles. Going to get an unbiased opinion tomorrow from a service manager at another dealership who a friend of mine knows.