[China Sea]

I know, this sounds strange, but I don't know why this is happening.
We have been trying to solve another issue and this one popped up.

They may be related, so I will tell you about both of them.

Our A4 starts fine and runs for a while (30 minutes at 1500-2000RPM) and then dies.
If we wait for 15 minutes, it will start and fun fine for another 5 minutes, repeat as long as you want.

Last year we had the same problem, but we discovered that if you kept the RPMs below 1200. it would run as long as we needed it (about 1 hour)

Over the winter I rebuilt the carb and the mechanical fuel pump.
Last year I replaced the fuel filter. To take it off the list of possibilities, I will replace it again tomorrow
Any Ideas?

While we were running at 2000RPMs yesterday, I opened the engine hatch to look at the engine and 2 of the "spark plug wells" were full of bubbling fluid (could be boiling water or maybe cooling water with exhaust gasses escaping out of ?
When the engine was running at 1500RPMs or less the "spark plug wells" returned to looking dry.
Background; Great lake 1965 boat, freshwater cooling.
Any ideas whats going on.? Thanks for your advise.