I have a 2003 Ram Van 2500 and on a trip this weekend, we had issues with the temp guage reading high and the "Check Guages" light coming on. I checked the antifreeze level and it was fine (to include the overflow). We were running the AC (it was warm this weekend) and I could turn the AC off and the temp would return to somewhat normal. Within a short distance (20 miles or so), the guage would read high and the "Check Guages" light would again come on. I recently (within the last month) changed the thermostat. I read to test the water pump, run the vehicle with the heater on and see if you get warm air. I did, so I'm assuming that rules out a bad water pump. I'm guessing it could be a sensor, but not sure which sensor it could be. Any assistance would be greatly appreciated.

 

If this was on the highway then it's time to replace the radiator. If this was in the city then it's likely the fan clutch is worn out.

 

I had a similar issue and fought with it for years. Came down to the truck i bought was a lease and had the radiator replaced with a cheap chinese aftermarket. it was very thin about .5 to 1 in. I thought I had all factory so didn't notice. Finally one mechanic spotted the problem and gave me a factory rad which was twice as thick. See if you have the same issue.