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imho.. coils measure fine …yet it is possible to test OK resistance wise…..while a low coil insulation spot allows the high voltage to jump to engine ground.
One way to prove this out would be to swap the coils to see the the issue migrates or suddenly disappears . you may want to do a close visual on the coil feeding the lower cyl to see if you have dark burn spots (assuming the coils are not black to start with 🙂 )
have you ever checked if the coil sparks will jump 5/16 to 3/8 across such a gap … at the very least using an old plug with the ground tip broken off the sparks should jump to the plug body with a SNAP !
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