2000 Ford Taurus Low/Fluctuating Heat output on 2000 Taurus

Hello guys,
Looking for a little help on a problem that is bugging me that I can't seem to figure out. I have a 2000 taurus with the 3.0 standard engine (not flexfuel or DOHC) and when car is idling at a light or at slower speeds the heater is only putting out a small amount of heat. When driving the heat comes back stronger but not to where it normally is. So far I have checked and has enough coolant, replaced the cooling fan relay (though was a problem keeping fans on), and checked to make sure ac was not running compressor kicks on for a second every minute or so but not running constantly. Within the last 2 years the water pump was replaced when I was replacing the engine cover gasket (figured I would while I already had it off), and the heater core was replaced as well (darn pain in the rear that was), and a new thermostat was put in. The coolant temp gage on the car don't fluctuate at all just comes to normal operating temperate it always does and stays there. For as long as I have own the car once heated up don't really seem to move. But so far that is everything that I though could be relevant and I know the car does not have a heater control valve so rules that out as coolant always flows thru the core. I am at a complete loss as to what would be causing that. I am getting a carchip to check the temp reported to the computer to see if there are any abnormalities there but not sure what would be causing it now. Any help would be greatly appreciated.