GradyB
Given everything you already tried
And the symptoms you described
I suggest you get a new coil
Either 6v keeping the resistor or a 12v version
Your symptoms follow a weak coil
And if you ran that coil too hard on 12v the internal insulation starts to break down and the output gradually shorts out internally so it became a weak coil
It takes more voltage to get a spark through dense air and fuel mixtures so that could explain why a weak coil works at idle but when you dump more air and fuel into the pistons the required voltage to get spark is more demanding and suddenly you loose spark
Oxygen alone is a good insulator so are hydrocarbons
You can draw a spark 10x longer through pure nitrogen vs oxygen
Something to consider.
-s
