1992 F250 Diesel running hot

I just bought a 1992 F250 non-turbo diesel. It has a stock 7.3 engine. The truck runs, pulls, and carries great under 2000 RPM. Below 2000 RPM the temp gauge stays at about 1/3 up the gauge (between 1/4 and a half). As soon as I go over 2000 RPM, the Temp gauge begins to move up, reaching just below the red line within 3-5 miles. If I drop her below 2000 RPM the water Temp goes down about as fast as it went up. It doesn't seem to matter if she is pulling, carrying, or empty. Her oil, and water/coolant are good. Her fan seems to be working. Any ideas?

Thanks,
Tim