my heater on my van not working right. It works but it dont. on 1996 Dodge Ram 2500

when the van is idling and tune the heater on that's just idling the temp goes up and up then when i am going about 35 or faster the temp goes down.
I have checked out the coolant it is full and I did flush it out very good. But when its working it works very good i just don't wont to over heat it gets that hot. A/C work very good and the fan is working on all speeds.
Thank you if you can help me Bryon

Asked by for the 1996 Dodge Ram 2500