Miss fires intermittently especially when hot

Converter needs replaced. What was the code?

misfires are more that often from faulty injectors and not spark related
check fuel pressure and injector condition
use fuel conditioner in the tank

Sounds like you're blowing a bit of oil through the cylinders. Pressure testing each cylinder will tell you if you have an issue.

What did the spark plug look like?Did it have oil or gas on it?

You will likely need new spark plugs and ignition wires or coils, depending which system is in the car.

Check spark plugs
check spark from spark plug wire
check fuel injector

check or relace your coil pack due to the fact that when a coil pack is going bad it usually looses its umph. when it get hot and that would explain random miss fires

Smell the coolant. Is there any hint of gas smell? Also look at oil, any coffee coloring indicates problems.

You can have car put on scope and it wll show any irregularities in the ignition system. It can also be dowithout any equipment. First, loosing ALL 8 spark plug wires, Now place the spark plug wires loosly back on, just secure enough to secure. You might need 2 people. Being the miss is more consistant in gear, APPLY THE EMERGENCY BRAKE , put trans into drive and raise rpm's till miss is obvious. Remove ONE plug wire at a time, TILL THERE IS NO DIFFERENCE IN MISS. Thats the cylinder missing. Start with the spark plug, inspect, work back to wires and cap. An EXTREME result will be NO COMPRESSION.

could be a wire crossed, or it's out of time. since you used motorcraft plugs, did you set them properly to the correct gap? replacing plugs without gapping them can cause the sluggishness, out of time can cause the mis-fires and rough running.