Windstar LX and gas smell with A/C on

I have a 2000 Windstar LX and often, not every time, when the A/C has been on for a while, I get a strong smell of gasoline in the car. The
last time it happened it was very hot out and the smell was very strong, even outside the car.
If the A/C is not running there is no smell at all.
Has anyone run across this or know what might be causing this?
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