What you describe is textbook for what I've always called vapor lock. Right, wrong or in between, the way vapor lock was explained to me by a 40 year master mechanic was that the problem is on the push side of the pump. Any engine at normal operating temp will give off enough heat to vaporize some of the fuel in the line between the pump and carb. The greater the heat the more fuel is vaporized. If the pump is too weak to overcome the vapor and pump enough gas to the carb you experience vapor lock. You are basically running out of gas. Eventually the fuel pump will fail completely. If you are seeing a line of bubbles rising from the diaphragm (the diaphragm is also the gasket between the pump halves) in the glass bowl you possibly have a failing pump. The best way I know of to check it is to rebuild or replace the pump.

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