Have you checked the fuel tank and clunk ? Pull the tank and, after disassembly of the tank, THOROUGHLY clean the inside of the tank AND the end of the clunk. While you have the tank out you might as well replace the lines inside the tank also.

All you need is a small piece of junk in the tank. It floats around, then hits the end of the clunk. No fuel to the engine. Engine suddenly stops, but you have plenty of fuel. With the muffler pressure off the tank and the engine not drawing, the piece of junk floats off the end of the clunk and the engine runs again, until the same scenario is repeated.