8hp Briggs & Stratton sputtering

my eight horsepower Briggs & Stratton is on a log splitter and it runs great until it gets hot it seems like.
then it starts sputtering when it hits a good load. I can move the governor back and release it easy and the engine will catch up and it will split for a few more minutes but then it just starts sputtering all the time then I have to stop it for about a half hour and come back and the same thing happens. it will split for about 30 minutes.
any ideas would be greatly appreciated. Thanks