[andyvh1959]

That reminded me of a tip my dad (old school mechanic) told me. Get the engine fully warmed up and have a water spray bottle handy to spritz a mist. Do this in the dark, no lights, spritz a water mist around the plugs wires and watch for any blue arching to the engine from the wires. An arch indicates a weak point in the circuit, like degraded insulation on plug wires, or a plug arching to the block instead of getting full spark energy into the combustion chamber.