99 2500 Ram heater/over heating

I have 80k on my Ram and it has runs great, but my heater fluxuates from hot to cold wile i have it in the full hot position. My eng temp increases as the heater cools and lowers as the heater heats up.

A diesel?

Gas V-10

temp goes up because it cant cool air, heater heater cools down because theres nothing to heat put a thermostat in it make sure there arent any leaks

???

Thermostat.

thank you