If there's a competition among responders then Rivets should win -- hands down -- he's a professional and I'm a hobbyist. :laughing:
You should read his post in the other forum carefully. He's given some detailed info about cleaning the carb.
I still think the most likely cause is some kind of clogging in the fuel system. Is there a fuel filter... and if so, have you eliminated that as a cause?
I don't have any real data to back this up but I'd bet that the symptoms you describe are overwhelmingly caused by fuel system clogging -- and things like low compression and a leaky manifold gasket would be way down the list. If you had 1000 of these engines with these symptoms, I'd bet that over 900 have fuel system clogging of some sort.
The degree of clogging could be sensitive to heat so it could vary as the engine gets hot.
Again, good luck.