Overheating at highway speeds

I have a '95 Intrepid with a 3.5 liter V6. When I am on the freeway (65 - 70 mph) after about 15 minutes, the car will begin overheating. I can bring the temperature down by turning on the heater. It does not overheat while driving around town.

I've replaced the water pump and thermostat and this made no difference.

The fans come on when idling so I assume they are coming on while driving on the freeway.

I am not loosing any coolant.

Any suggestions?

Pete