My heat only works when i am parked on 2000 Mercury Sable

My heat works when I warm my car up the engine gauge goes to warm but when i start driving the gauge goes down past cold and the blows cool air?

Asked by for the 2000 Mercury Sable
Sounds like a possible engine thermostat opening too soon and staying open. Check there first.
One thing I would check is the anti-freeze. If you have added any, it may test too low. In a lot of engines, too high ratio of anti-freeze to water will cause a heating problem. Testing at about 35 below zero is a good target. I hope this helps.