I have a 2000 F150 4x4 with the V6. This thing has been giving me a headache with high coolant temps. I initially thought they were false readings, so I changed the sensor. I had no issues through the winter, but now that I have reached summer in Alabama, the gauge heads up when climbing grades, especially with the air on. Running level with the air doesn't seem to bother it, but hit a grade with or without AC and you may get a hot reading all the way to red.
I have replaced the bottom radiator hose, this required fresh coolant. Replaced the cap. Ran 80mph on the highway with the AC blasting , and all was well. Hit a grade and the gauge went up. The thing has been idling in the drive for 30 mins with max AC, and all is well. What gives?

 

Have you checked for build up between the radiator and the condenser. Had a similar issue with my 4.2, and there turned out to be a bunch of crap down between them from where I had been parking and driving.