92 Ford Escort running hot

My question is why does my escort run really hot when I drive on the streets and when I drive on the highway it stays cool. But if I drive on the highway for more than an hour then it will start getting hot. I`ve recently flush the radiator and I am thinking of changing the thermostat. The book calls for a 192 degree thermostat and I was thinking of putting a 180 degree thermo stat in. I live in southern california.[/B]