Have you checked the HT wires at the plug cap end? They corrode & come loose with age. I was able to unscrew all 4 of my caps, chop out the corrosion & screw the caps back on. Threads in the caps can be corroded too. Don’t think they will be failing at the coil end as they are sealed but if the seal has broke down you can possibly do the same thing?
Also check the 2 wires & plug that go to the offending coil.