Chevy Silverado is fouling out all 4 spark plugs on the right side. Johnsonjay

My 1997 Chevy Silverado (5.0L Vortec) is fouling out all 4 spark plugs on the right side. I have cleaned the throttle body, replaced rotor, cap and wires, mas, fuel filter. It runs great when i get down on it or pull my boat, but if it idles or kicks into overdrive cruising around 1500 rpm it starts to spit and sputter and miss etc, it also will leave a black soot spot where the exhaust hits when stopped.
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