Feets,

You might look at some sort of blockage in your radiator. We have been through this issue twice with my wife's Durango, and it is a far more common problem than most people realize.

Usually if it is the radiator, the vehicle will only heat up at speed, much like an airflow problem due to improper ducting through the radiator.

I have seen this happen with radiators which looked to be in great shape visually, and some which were only a couple of years old. Older cars driven only locally around town are notorious for it.

May God bless you and yours,

Paladin