Overheating 1991 Mazda Protege

The car is a 4 cyl 1991 Mazda Protege. This car has about 96,000 miles on it. It is overheating when at an idle or very slow traffic but does great on the highway. So far the radiator has been flushed and thermostat changed. The temp gauge was checked to see if it was faulty but it isn't. It appears to be circulating water ok but can't say for sure. There's no water on the outside of the engine or in the oil. What do we check for next? Any help is greatly appreciated. Myson bought it to take back to college and he is very mechanically challenged but it seems to have our local mechanic stumped.