Re: The engine ran great!!!….except when it didn’t.

[Don]

Seems like there is some sort of ignition issue, possibly aggravated by moisture….
I guess I would do a spark check on it when you start it and it is running like crap…Perhaps use a timing light or those inline neon testers to figure out which cylinder is misbehaving…
Not for nothing, and perhaps I skipped over it in your post, but have you tried new plugs? Just cuz the ones you have in the engine are relatively new, doesn’t mean they are any good…
The only other issue I can think of is if this engine has a stop circuit/key switch, or perhaps one you have added to the wiring…If so, you may want to disconnect the leads (if that can be done easily without removing the flywheel) and retest…