the fact that your temp rises when you turn off the fans on the highway suggests the airflow is a bit restricted but you are NOT overheating & as said you are too cool (at slower speeds) which drastically increases eng wear. I'd replace the stat with another 180 in case this one is defective & if not then a 185 or 195 to keep the temp at 180 minimum as said. I believe you want the water speed to be as fast as possible as long as the pump ain't cavitating which is not the issue here