My 2007 dodge ram 3500 6.7liter is overheating once it gets into the overdrive.

What is your question?

After it overheats it cools off quickly while the truck is still running. It runs fine as long as I can keep the truck out of overdrive. It overheats a little quicker when I have a load on my trailer.So far i changed the waterpump, thermostat,fan clutch,radiator cap,flushed radiator and bled the system.I also checked the fan belt and it seemed to be a little play, but im not sure if that is normal or not.