It sounds like a vapor lock issue, although this isn't a common problem anymore. Vapor lock is caused when fuel in the fuel line turns from a liquid to vapor making it difficult for the fuel pump to deliver fuel to the engine. The fuel can be vaporized after being exposed to hot temperatures. It may be worth checking the operation of the engine cooling fan to determine if excessive heat is causing the problem. I'm not an expert, so I hope someone more knowledgable can weigh in. Good luck.