it sound like if its building up pressure there is a venting issue prob a plugged hose the pressure changes the properties of the fuel making it volitile to boiling

 

if you not venting properly its definently gonna bottle up heat. The same happens to a gas can when the sun hits for a while. Maybe try blowing the gas cap out with an air hose to get rid of gunk or something that might be in there.

 

I had the same problem on my popo. The deal is that riding in hot temps at low speeds the gas tank gets too hot from the radiant heat off the engine and exhaust.
To fix this, I removed the factory heat tape off of the gas tank and cleaned it very well. I then installed some stick on heat shield on the bottom and sides of the tank. I covered a much larger area than the factory did. I covered as much of the tank as possible without removing the front plastic.
This seems to have fixed the problem.
Design Engineering DEI 010462 - DEI Reflect-A-Cool Heat Shields – SummitRacing.com