2000 Ford Taurus With No Heat

Hello;
My Taurus doesn't put out any heat. And with another Minnesota winter coming, I need to get the situation fixed asap.
So far I've had the coolant system flushed and the thermostat
replaced. I've had the heatercore checked and it is fine and not clogged. I've also had the fan, vents, and the box in the dash that blends the hot and cold air (sorry for the lack of correct terminology) checked and all are working.
When running, the engine temp climbs and stays at a normal operating level. (gage reads right in the middle)
When I go down the highway, the heat will climb to luke warm at best, and only after about and hours worth of driving. When the vehicle stops, the air goes immediatly back to cold.
Any ideas as to what the culprit is would be greatly appreciated.
P.S. It's the 3.0 without the DOHC.
Thanks,
Rondo
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