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.

I'm on my second after market gauge (running a good autometer currently) and they were both reading the same so it's definately not the gauge. I would try playing with the heater but the damn cable that goes to the heater valve is broken...