We cleared a code for a catalytic convertor on our 2005 Expedition that has 76k miles with a 5.4l gas engine. The local Ford dealer replaced it under warranty and informed us that they also found a misfire code. They wanted $785 to do a standard tune-up. We passed and asked our inde to install some new plugs for us. First of all, the good ones cost $80 for a set. Second of all, the dealership service advisor told us to be sure we also change the rubber boots at the same time. Does this make sense to anyone out there? The truck runs fine, unless you ask for full throttle at which point, she acts like there is a banana in the tailpipe and the check engine light flashes. So I turn off the truck, and upon restarting the light stays off. The average person would never know about this. What should we do at 76k miles? Plugs only...plugs and boots...more?...less?