you may have lost compression. this is due to a burned out piston and sleeve. mine did the same, easy to start cold because the engine was "gummy" with lube. then when it heats up the "gum" is gone and there is not enough compression the run the engine at low speeds. You need to get a new piston and sleeve for about $35 and rebuild the motor. then try not to "lean" the engine as much. I just rebuild my engine 3 weeks ago and it is like new. i am now working with a "temp gun" to keep the engine temp around 220 degrees to ensure that is lasts longer