Naturally the carb fuel inlet isn't connected to the tank vent versus the fuel outlet, right? It's happened.. with remarkably similar symptoms. Also, have you given the muffler pressure system the same attention as the fuel side? Cleaned the pressure tap, replaced the pressure line, etc.? If in doubt try tuning up the engine without the muffler pressure line attached, level on the ground/bench. If is now consistent.. aha. Not likely but just offering a possibility.

Was the glow plug replaced before you tried running it again and is still in there, or after you started having problems? Just a question in lieu of fuel delivery issues.

Engine that go lean after running for a while have been known to develop air leaks as parts heat up and fits change.

Are your head bolts tight? Plug gasket installed? i.e. everythin sealed up ship shape up top?

MJD