Vapor lock occurs when fuel in the fuel line "boils so to speak" somewhere in the fuel line usually between the pump and the fuel tank. The causes can vary from overly warm fuel which can quickly turn to vapor in the fuel line when the fuel pump applies a vacuum to the intake line which causes the fuel to instantly vaporize and therefore the pump quits pumping or perhaps the vent on the fuel cap is plugged or maybe someone changed the cap to a none vented cap which will lead to a vacuum in the fuel tank which in about 3 hrs could become quite severe to the point of stopping fuel flow by once again causing the fuel to flash to a vapor in the line leading to the pump from the tank.
I would check the fuel cap for proper venting. I doubt that fuel temperature is the cause in this case as there is no return of fuel from the carb to the tank. I suppose another cause could be some junk in the tank plugging the outlet leading to the same problem.
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