Subaru Forester 2001 hard to fill with gas when weather is warm

When waather is warm I have to put in gas as slowly as possible. and it still shuts down.I called the dealer where I bought the car and they said they never heard of this problem that I probly needed a new gas tank? Now that the weather has wamed up I no longer have this problem. I thought when we started using unleaded gas they put in expansion tanks so no fumes would escape? could something like this be the problem?