Jim,

Do you encounter miss fire on the stand when you on the ground on the stand?
Try to do the same test and go to 7.4v. This will put more power out of CDI.
It could be 3 things.
1- the High compression of the engine may kill the spark and the spark will jump to ground in the cap or inside. 7.4v should give more voltage.
2- the spark plug style . With exposed Portelan or NOT. Some times this makes a difference. Gap also need to be right. See if you can find the right gap that Sthill recommends for that engine.
3- Carb adjustment, May act different in flight than on the ground. If the metering diaphragm gets bothered by the air around the hole.
Some people solders a little brass tube on the Metering plate and runs a tygon tubing in to the fuselage .
It could get rich, then high compression will make it misfire.

Those will be my thoughts on this issue.

Adrian