1996 Jeep Cherokee Overheating

My Cherokee ran normally for about 4 hours on a trip. Temp gauge hovered right around 195. The first 4 hours were on secondary roads at 55-65 MPH. On entering the interstate, I accellerated to 75, and within 15 minutes, the temp went up to 245-250. I exited the interstate and slowed to 45-50 and the temp came down to 225 or so. When I reentered the interstate and sped up again, the temp started to rise again. I found that by modulating my speed, I could keep the temp within limits. I had to slow down on hills, and could speed up going down hill.

I have replaced the radiator, water pump, all hoses, thermostat and serpentine belt. I added a Flush N Fill tee to the heater hose and "burped" out all the air. But the symptoms remain. The fan clutch appears to work OK.

What do I do next?

Thanks.