My 1995 Mercury Sable stalls on hot days (over 90) if I have to run too many errands.?

My 1995 stalls on hot days if I have to run too many errands. The temp gauge does not say the engine is hot. It's okay for short errands. But if I have several stops to make, or if the air conditioner is on, it will stall. I quit running the air conditioning, and it still happens. If I let it sit for about half an hour (after stalling) everything is OK.