Car stalls only when gas tank is half full

I don’t know the protocal about answering but here goes…thank you for your intelligent and quick answer. I have been to three mechanics and the dealer and have not gotten any diagnosis or solution that was anything but, at best, a guess. Now I will try and find a mechanic that will run this down properly in that I feel armed with adequate information. I thank you so much. Sincerely, Judy