1999E250 5.4 gas only engine a little rough idle but extremely poor acceleration

Hello, I'm concerned with "guesswork" on this, I'd really prefer to have a proper test performed to see what trouble codes exist. If you have poor acceleration, it could be ignition or air intake related, or even fuel delivery related. Have you had a relative compression test performed? This would ferret out any coils or spark plugs you should look at. Incidentally, I'd ensure all plugs are replaced with proper Motorcraft plugs, and all coil boots are replaced when you do that. If rough running continues, you may want to perform a base engine compression test to see if there is a base engine concern.