my walker EFI mower runs good when the weather is cool but on 90 plus days if run continuously, over and hour or two on a large lawn it begins to loose power then sputter and eventually die. it will restart after cooling off. however to be able to run for any large amount of time after this it has to cool off for an hour or two. The over heat light never comes on. I blow of the engine and cooling fins every day with the leaf blower. Any ideas?