I have 99 F150 with the 4.2 . It overheated last year so I changed the thermostat, all is well again. This year the heater hose was leaking , changed the hose and all is well again for about a week, then yesterday I felt like I was lacking power, sure enough temp gauge is climbing , so I hop on the highway quick and get going, temp gauge went to normal and stayed normal the whole way home ? Is there a bubble in the coolant ? Fan clutch ? 160,000 miles on it .

 

I'd expect the fan clutch but I suppose an airlock could have been the trouble. How hot was it that day? I believe that highway speeds can move a lot more air over the rad and pretty much negate the need for the fan clutch while at high speed, but I guess the acid test will be to drive in stop-and-go city traffic on a hot day and see what happens with the temp gauge. Good luck.

 

The missing part of your test was at the end. If it started heating up again when you sat at idle after you got home, or after you got off the highway, that would be a clue. Reproducibility will tell you something.

 

Drove it again today, 82 degrees, drove for 20 minutes on the highway and ten minutes idling after that and it never got hot . AC running .