WEll in my wifes Durango only gets warm when it's sitting still. Once you start driving it gets cold, Also it you turn the heat up when the truck is sitting it will get colder as well. Do you think this could be a bad heater core as well or air in the heater core?

 

Could be low on coolant or even a partially failed thermostat.

 

but let's be fair, there are times when it amazes me in a put a smile on your head kind of way like when a spark plug completely backed out and it ran just fine and were it not for the loud band along with the nice dent from the underside of the hood as it ejected, I probably would still be driving around on 7.