I have had this problem for about 2 years now, where the temp gauge spikes to the top. While the engine is under load, if I release the clutch and let the truck coast for a bit it starts coming down. It usually happens in the late spring or summer when the outside temp is hotter (of course) I live with it because I rarely drive the truck in the summer. But what I have noticed is that when the heat is on the gauge never spikes, I have replaced the thermostat twice with a flush and fill each time and have replaced the reservoir cap but the problem persists.
I'm wondering if the radiator may be clogged, but I have pressure on both sides when the engine is hot (testing only by squeezing the hose)
looking for help from anyone who had a similar problem, or may know of a solution
thanks

 

Sorry I guess I should mention it is a
97 F150 XL 4.2 with air 2wd

 

those guages arent always accurate, you can get it tested with a known accurate guage and go from there