Could your guage be lying to you? I had a fan clutch go bad once but I would think at freeway speed you would have enough air flow anyway. Other than that maybe you are running lean but I would think you would be getting a code from that...
Play with your heater and see if there is any variation. I had an '87 22re once that would get a little hot at highway speed and when I turned the heater to full hot and back to full cool again after a few seconds it would snap out of it and cool down. Never did figure that one out.
