Over heating on the freeway?

Hmmm..... the car cools fine at low speeds, but overheats on the highway. Is it possible that you have a "pusher" fan installed on the engine side of the rad, and when you drive highway speeds the fan is fighting the normal airflow? Just a wild *** guess Cope, but I've seen stuff like that accidentally happen even at the highest levels of motorsport.
Please let us know the outcome of this, it's really interesting.