Hi XXXXXXXXXX XXX. Thank you for asking your question on JustAnswer.
It sounds like a secondary ignition misfire.There really isn't any good diagnostics on this vehicle unless you know someone with an old engine scope and can get it to miss right there in front of the scope.I used to water test these,I would get a spray bottle of water and spray the plug wires while some one was lightly brake torquing the engine,I would also spray the distributor cap and then the ignition coil and wire.Usually the faulty component would show up as the water provided a direct ground path from the bad wire,cap or coil.This works well for cracked spark plugs as well.
They were not kidding about tuning this engine.The procedure is pretty strait forward,just remove the interior engine cover and the air inlet and filter housing from the front of the engine.Then remove the plug wires,cap and rotor and the plugs. The hardest part is getting to the wires and plugs....there are no short cuts.You are basically cramming your hands and arms in very confined space.It can be done and you will need every combination of double swivel plug sockets,flex head ratchets wobble tip extensions of all different lengths.
Ford Technician
Ford Senior Master,Ase Master with Advanced Level,25 year Ford Tech, Auto Trans and Drivability,Etc