Ethanol is a bit of a disaster when used in engines that are not run for extended periods (think lawn mowers, snowblowers and other seasonal equipment), but I've run my daily drivers on E10 for years with no issues. With your regular use of Gumout, I doubt that fuel system deposits are your issue. I believe that your 4.6 uses a coil-on-plug ignition system, giving 8 opportunities for a misfire. If the plugs have never been changed, that would not be a bad idea, just make sure that the engine is ice cold before removing the old plugs, as Ford's 4.6 and 5.4 engines have very few spark plug threads in their cylinder heads, and stripping the threads by removing spark plugs from a warm engine is not unheard of.