All this time I was thinking the problem was heat and ignition because of high air temperature and previous ignition trouble. Every time I changed something in the ignition (batteries, switches, plug gap) it ran better. It was running perfectly when all of the symptoms suddenly returned. With the cool weather I could now tell it was going lean (then quitting).
On the fifth trip into the carb I noticed what appears to be an anti-syphon check valve between the regulator chamber and the needle valve supply passage. The flapper in the check valve is gone so I'm guessing it parts are plugging the high speed discharge nozzle. I have installed a different carb and have yet to test fly it, but here's hoping this will be the fix.

Jerry