1993 Honda Civic wants to stall

My problem is when I have less than a half of tank of gas and it is about 90 degrees or higher the car wants to stall at idle. When it is cooler out the car runs just fine. My buddy told me that since it is a closed loop fuel system, that there is a one way valve connected either in route with a rollover valve or in the gas cap, that is not letting air back into the gas tank. Therefore when it is hot out and not much fuel in the tank, the fuel pump works too hard and shuts off. Im not quite sure where to go with this one. Any info would be helpful. Thanks.