i have a 1996-97 f150 with a 4.6 ltr triton engine when i first start my truck i let it warm up to operating temp according to the temp gauge i begin to drive my truck and the heater inside the cab begins to go cold then tempgauge indicates cold i continue driving for 30 -40 seconds and then temp indicator jumps all the way to the hot the overheat indicator on the dash lights up and the engine derates i pull over shut the engine off and start it back up when I do this the temp gauge registors normal operating temp and the over heat light turns off the truck then operates normal for the rest of the trip until I let it cool off again I have changed the thermos stat

 

I found three codes that seem to pop up together they are p0500, p1299, and p0420. One is an engine overspeed code(p0500), the other a head protection code(p0500) and the last a catalyst failure code(p0500). Does this information help?