Small Engine Troubleshooting Problems? Ask a Mechanic Now.
The thought here is tat the coil is going bad and "opening" when hot and "closing" when cold. Go to the local auto parts store and purchase an "inline" spark tester and check spark when it stops running. If there is no fire, replace the coil/solid state.
Another thing to check is make certain the hole in the gas cap is open allowing venting of the tank and eliminating any chance of a vacuum. THANKS