2002 Chrysler Concorde car tries to over heat

One time, I was having fits trying to repair this exact problem, everything on the car was new and still once in a while the gauge went hot, slowly returning to normal.
After a couple days of pulling what was left of my hair out, the service manager brought a bulletin, not a service bulletin but just a managers eyes only engineers notice about a few cars coming through like this. It was just a ground resistance equation and took 5 minutes to fix, is the coolant level ok? It may be the thermostat sticking or its low on coolant, even the fans that cool it may be bad or their controls, air in the system from a leaking head gasket, all will cause this also, it may just be the sending unit circuit also.

I have the very same problem, can you please explain the "ground resistance equation" and what you did in the 5 minutes.

Thanks in advance!!

I have the same problem except mine will slowly heat up just like that but it will only go down if I have the heat on full blast

First question, check between the radiator and the a/c condensor, make sure it is cleaned out and the air can go through the radiator easily, {had it happen to me} If leaves and pine needles get in there they cut down on the air cooling the engine. Even though you put a new thermastat in does not mean it is good, have seen some after market parts being bad even though they were new.
With it beining a 2002 you might want to back flush the system from any containments that might clog up things. It also may be the temp sending unit is going bad.