The fan Steve posted might be your salvation.

If a car runs down the road at any speed cool, but comes up when sitting at a light, it is because of insufficient airflow. The fan has to duplicate what you are getting when underway. Mine runs at 150-160 going down the road, but if I don't turn the fan on at a light it will start to creep up. When I turn it on the temp goes right back down to 150 or so.

Don