Strange temp problem
 

Just took my 83 on a 1800 mile trip and noticed the temp gauge acting funny. The temp gauge would be pointing at the bottom of the T in temp which is about normal but then slowly go up and get pretty close to the line before the red zone. A couple times the needle went right back down to it's normal spot rather quickly and other times it wouldn't. Most of the time I'd just let it go and by the time I had to fuel up, it was just holding steady. This was traveling on flat ground, 70 mph on the highway, at night or day, A/C running or not. Going up long or steep hills it was hit or miss when it acted up.

Now here's the really weird part. Thinking this thing is going to overheat on me since the needle went right close to the line, I pulled over and let it idle for a few min checking things over. Look at the temp gauge, hasn't moved. Raise the idle just a little to get things flowing better, no change. Turn the truck off for 2 min and restart, the needle points at the bottom of the T like normal. I did this again when I got home and the temp was reading high again. Turned the truck off and imediatly started it up again, temp gauge went back to normal. Seems like it resets whatever is screwing up because I can't see how it can go from running hot to running normal in a matter of 5 seconds. I did the same trip last year and it never acted like this.

This sounds like an electrical problem to me and the gauge is not reading properly. I'd try to put a mechanical gauge on there and get a true temp reading. That'll tell you for sure if something is wrong. The sending unit could be going bad or the wire from the sending unit shorting out somewhere is my first guess. If there is a true overheating problem my first guess is the thermostat. I'm sure you made sure it was full of coolant right?

Yep, sure did check the coolant level. It was to the proper levels plus the radiator is just a tick over a year old now. Thermostat was my first guess too but doesn't explain why the gauge reads normal again after shutting off and starting up within seconds. I have a thermal gun I could carry with me to check temps from when it's running normal to compare when it shows it's heating up. Only problem is it takes about 80 to 100 miles of highway driving before it starts to act up. It never once acted up just going through town or when I did a short trail run (Box Canyon) while in Arizona. Temps there were 107 ish that day. That also included about 35 miles one way of highway driving at 70 mph to get to the trail start.