Jeep wrangler rebuilt motor overheats at 65 mph?

I have a 2000 jeep wrangler with a rebuilt motor (6 cyl), new water pump (high performance), new thermostat, new belt, new radiator and new high per. oil pump. It runs fine until I reach about 65 mph and it starts to get over 210 degrees (where is has alway stayed, even before I change motor). I would say it get about 250 degrees, never hits the red, but I usally don't drive it more that 20 miles when it starts to run hot. As soon as i slow down it gradually goes back to 210 degrees. the temp gauge is working fine. Does anyone know what it might be?