Frank_Ford

Do you have oil, grease, dirt in the plug socket? Sometimes this will prevent getting a good seal on the plug. You can clean with WD40, rags, and a round wire plumbers brush.
Also, is something like oil or coolant leaking into the plug socket? This also needs to be fix as it can cause misfires. Use di electric grease on the electrical connections and see if this helps (cost about $4 at autozone). Good luck