AC blowing hot after running awhile

On trips longer than thirty minutes on the expressway my AC goes from cold to hot and will not go back on. Sometimes (rarely) if I stop and restart the car it will go back to cold but for the most part I cannot get AC again until the car is completely cooled down or next morning. What part needs replacement? Freon is good and there are no leaks per my mechanic who said he could not recreate the issue. Of course not because he dose not drive it for long distances. Help!