On hot days the engine will barely come back on after stopping for gas.

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After flooring the gas pedal, the engine starts back up and idles really roughly, sometimes shuts off. After a minute or so of revving it a little, the problem goes away. On really hot days it shuts off by itself at intersections when it should be idling. Could it be some kind of vapor lock in the fuel line? Or a computer or sensor problem? The overheat light comes on at erratic intervals, even if the engine is still cold. We replaced the temperature sensor for it already. Also the fan for the air conditioning seems to run fast/slow, fast slow. No diagnostic codes about it, several car mechanics have not been able to find a solution.