I have a 1992 Dodge ram 250 extended cab long bed 4x2 with the 5.9 diesel in it with an automatic trans, every time I pull my 27 ft boat up a slight hill it wants to start to overheat. I got one of those hand held thermometers and it shows nothing over heating in the head area, block, radiator etc... all shows running normal. There is however a relay that gets very hot, if the temp gauge shows to hot it will shut the engine down for a minute, then it starts right up and slowly cools down until the next hill or continue of the same hill and continue to overheat

 

Make sure the fan clutch is working, you should be able to hear it come on. You may have to clean the radiator, inside and out. IS the thrermostat working?

 

You bet I can hear the fan, it's got a good roar to it when it starts to overheat ( according to the gauge), I changed the thermostat to the factory 192 degree and I recently had the radiator cleaned, Thanks

 

It's possible the gauge is bad. The factory gauges aren't the best. Try an aftermarket gauge.
Is it losing coolant when this happens?
Do you see any bubbles in the radiator? (headgasket)