The model number info confirms that we don't have ready start. With that eliminated as a possible problem i suspect that you may have an ignition module heat sink issue. Next time it doesn't start, remove the spark plug, connect it to the plug wire, and hold the plug against a good ground on the engine, and have somebody crank the engine and check for spark.
Ignition module may be cutting out when the air flow over the engine stops, and the heat off the engine rises, shutting down the module.