Can someone explain why when my 1998 5.2 Durango gets hot. The heater blows cold air. And when it cools down. It does it within 5 min. heater blows warm air again.
I have new water pump and term.

Did you bleed the system properly?

Did all this just start after the water pump and therm(ostat?)
First, make sure everything else is working properly then, let it just cool off. Overnight if possible. The next morning, overfill the coolant jug before your first drive.
By the time you get the first drive cycle complete, all the air should be out of the cooling system and all is well now.