My heater blows hot when you first turn on the fan. After 5-10 seconds it starts blowing cold air. Its worse on the drivers side than the passengers side.
If you turn it back off and wait for a few minutes and turn the blower back on. It is hot again for a few seconds then gets cold again.
I'm not sure how the blend doors work. But air blows out where it is supposed to. Just doesn't stay hot.

 

Both heater hoses hot? Coolant level correct?

 

This^^^^

Then maybe..........

http://heatertreater.net/dodge-ram-02-08/#

 

Sounds like possibly the hot/cold blend door if you have good heater core water flow.

 

Exact same issue, what was the fix?