Has any work been done on the engine? Especially the carburetor? There is an insulator piece between the engine and carburetor to reduce the heat from the engine. Also the muffler has a screen inside of it, if you have the two piece muffler on the engine that gets plugged up and the motor looses power. There are also two small feed tubes that hang down inside the fuel tank that also get gummed up and they also will cause the same symptoms. Is the motor burning any oil yet? This will happen when the rings get worn out. If the valves get bad the motor will be hard to start due to lower compression. There could possible be a mechanical cause from the cam shaft if it lost oil to it for some reason and caused a lobe to round off, but this would be prevalent at all times. You can put a compression gage on the engine to check the compression by pulling the cord and rotating the engine over three compression cycles to get a reading. Good Luck, RAC