Sorry I guessed you might not have known what you were doing, you've cleared that up. The way I like to check spark is with a surplus plug that I cleaned then opened up the gap to about 3 mm. If you open it too far it won't spark across the gap even though it will work in the engine.
In my experience it is not uncommon for sick coils to work intermittently for a while before they die altogether.
The key point to check before buying a coil, is that the kill wire on the old one is not somehow becoming grounded when it shouldn't. That can happen due to problems in the sliding tin bits of the governor where it grounds the kill wire when the speed control is at minimum. It can also happen upstream of there along the wire as Joe said. The diagnostic history does seem to be coming together, though, if it would start on Aerostart when the spark was working, and you know that now the spark is not working.
