My experience similar to yours turned out to be crud in the tank, line and carb. Not enough to keep from running but enough to make it real hard to start and make it run hot due to being lean. Buy a new plug but check the fuel and carb too. Get a good clear glass jar and let all of the gas drain into it and let it sit for a day to see if you have any sediment in it. Do it before the filter just to be sure that it is there or not. Then if it is, clean and flush the tank, get a new fuel line/filter and clean the carb and all of it's orifices.
Good luck
