Windstar heater problem

I can't be the only one with this problem. My '98 Winstar will only blow hot air when the vehicle runs above 2k RPM's. Example: While I'm moving down the road I get hot air. When I'm stopped at a red light, stop sign, etc., the temp slowly declines to cold. Rev up the engine and the heat comes again.

I've read alot of the posts about the blend door but the symptoms don't indicate that (no thumping of the door, the blower switches to defrost, floor, mix, etc.).

Give me answer please!

sounds like poor circulation to me. When was the last time you had a coolant system flush & refill?