Overheating & heater Issues

What is your question?

We have a 2000 ford ranger that overheats then gets cold. the heater does not work until the truck overheats then starts working until it cools off then it overheats again and the heater works again? The gage fluctates from cold to hot the entire time you are driving it. We have replaced the thermostate and flushed the radiator. Are there any other suggestions as to what is wrong and how to fix this problem?