Classic symptoms of bad plug wires, especially if they're the radio frequency suppression type. Check the resistance of the "wires"- - -they should not have more than 1,000 ohms per inch. 12" wire- - - -12,000 ohms max, etc. Those wires are notorious for getting open-circuited spots that "grow" as the spark fires inside the insulation. I consider 30-50K miles to be the maximum lifespan for RF resistance wires.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!