1998 Ford Windstar heater fluctuation

In driving my van after it has heated up (temperature gauge showing heat), it will blow hot. All of a sudden it will blow cold, sometimes if I slow down, but not necessarly. Some time latter it will blow warm again. All of the time the rear heaters temprature will stay constant. I have a full radiator and overflow tank, and have gone through several (10)cycles fo heating and cooling off to eliminate air pockets hopefully. The heat at idol (wating for the kids to get out of school ) is also tempermental. Any sugestions?
Thanks