Close the coil air gap to .008, it may make a difference.

I thought I'd find the valve settings on google, but no luck. Do you know them?

Maybe a valve seat is riding is riding
up, upon warm up?

Then it would have to reset itself because within 5 minutes of shutdown, I had the valve cover off and there was plenty of gap.

.004 each will do on these. If compression seems good when it shuts down, it could be a fluke coil. I assume a shot of carb spray fails to revive it? The only thing I've seen similar is the gummy valve stems. Once it starts, follow through w/ bursts of carb spray to help clean them.

Start with the Simple Bay , (KISS) You know the Program . Start as Tom advises with the Carb Cleaning Route and (or) Top End Lubricant cleaner should the Valves look suspect ? Then perhaps Coil Load Testing or the Baking Trial . Just a Question , Compression and Spark Plug are Optimium ?

I siliconed the valve cover back on today and will check the coil air gap, spark, and carb trick tomorrow. I just love engines that use silicone instead of gaskets on their valve covers.

I guess they're trying to tell us that their engines are so good, you never need to remove the valve cover.

Update: The unit is running. I tried the carb cleaner and not the coil adjustment. I started it and intermittently sprayed carb cleaner into the throttle body. After about 5 minutes, it kept running. (It never threatened to stall) At that point I added an ounce or two of sea foam to tank 1/4 full and put it aside where it ran for about 30 minutes without stalling. Case closed, for now.

Bay: Good to hear you seem to have the problem solved. Should it reoccur , try checking the plug colouration if lighter than normal check for intake gasket leakage . The back firing scenario may indicate sucking air , but also can be from carb related cleaning requirement as you have found.