My 2001 ford ranger 2.3 liter manual trans, temp gauge gets hot whenever i go up a hill during hot days only.?

during winter i have no problem going up the same hill, heater works good. i have changed thermostat, had radiator cleaned and reconditioned, changed water pump and replaced hoses. it does not leak coolant. but whenever outside temperature reaches 90 or more going up this hill temp gauge gets hot then goes back down when i start going on level ground