How old are your spark plug wires, plugs, cap, rotor, and coil? Stock 5.0 ignition equipment really sucks!
I had the same problem in mine shortly after I got it. It had some old ford racing 8mm wires, a generic aftermarket coil, and the worst aluminum cap and rotor I have ever seen.
I got an Accel Ford Super Coil, Accel High Performance Brass Cap and Rotor, Accel 8.8mm 300+ Racing wires, and Accel Copper U-Groove Plugs. Not only did it fix the high RPM misfiring, but it felt like it had increased throttle response and more power then even before it started misfiring.
Oh yeah, and when I pulled out the old plugs, apparently the guy who had it before me didn't believe in setting the spark plug gap, because it was WAY to big!
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