Based on what you are describing and the age of the engine you might want to start with the following:
- Put in fresh gas and a fresh plug. Don't assume what you have is OK until you prove it.
- Clean the carb and make sure the pilot jet and air jet passages are clean. When gas gets old it will tend to plug up these small jets.
- Check the valve adjustment. Once the valves start to tighten up it will be hard to keep the engine running properly.
- Check the engine compression. It's a 13 year old engine, it might just be worn out.