1996 Ford Windstar engine run hot

Driving in winter is OK. Over heat in hot days, specially if I have to drive slow, the temp goes up until the car stall (happens one's). The fans are both working, I check them out many times. I flushed the Rad and change the thermostat. I am managing to control the engine temperature, turning the internal heater at max. As you see, I am cooking my self inside this oven with wheels. Thanks.