I think it's from being too lean, could be needle valve is dirty or an air leak.
Could be a leak in your tank or fittings, a pinhole in the fuel line, crack in the engine casting, or a poor gasket.
My CMB .45 was acting goofy and it was a slit in the fuel line at the fitting.
You probably have but double check your throttle servo and linkage to be sure it's working properly.
We always run pressurized tanks at our altitude but I guess at sea level you don't always have too.
Good luck hope this helps.