I know this is a truck forum, but I had the same problem on a 99 Taurus, plus I own three ford trucks, so should be able to get a way with this. This is going to sound very strange, but let me paint a picture. The issue with me started with leaking freeze plugs, took it to ford, they replaced them, once they replaced them, the heater issue started. Same issue, cool at low speeds and idle, but hot at highway speeds or high rpms. Did a lot of research on this, found out that the reason the freeze plugs were leaking was due to corrosion in the radiator due to wrong anti-freeze. This resulted over time in eating away at the freeze plugs which caused them to leak. Not just one or two of them, all of them. This also resulted in eating away at the fins on the water pump which resulted in low coolant flow, the faster you go the more flow. Again, I know this sounds a bit strange, but this was my problem. Also the car had a lot of static electricity which also results in electrolysis; the reason boats have anode plates. During the cooler months, you step out of the car, it would shock you like no one's business. Research also noted this to be an issue; the electrolysis would cause the same effect, eating away at the freeze plugs and water pump fins. I was not sure which was the cause, but one of them or both caused this problem. Short answer, check your water pump.. Hope this helps.

 

you can take off the heater hoses and connect a garden hose to run thru it. see that you have full flow... THe blend door takes hot air from the heater and cold air from outside and mixes it to the temp you have adjusted to.. If the door is broken or flopping around, the temp can vary.. Take off the plastic trim under the radio and above the transmission hump and look for the motor and bell crank. I think you can lay on the drivers floor and look up above the gas pedal at the radio and see the motor also.

 

K thanks for the advice

 

There's a switch that activates A/C and/or heater...replaced my daughters today now they both work...could be your problem.