i have a mercury marquere. when i drive going about 40 miles per hour it is ok but when i get up to about 60 miles per hour and drive about 40 miles it starts blowing hot air. but as soon as i slow down to about 40 miles per hour it starts blowing cold air.
The actual cause can vary a bit based on the year model of the car but this sounds like a blend door problem.
On older cars this was usually caused by a vacuum problem and on later model cars may be caused by a stepper motor fault or a stepper motor controller problem.
Should have added something. If the temp change is gradual this could point to an abnormally low refrigerant charge. At higher speeds the low side pressure may be dropping enough that the evaporator core temperature is at freezing or below. This will ice the evaporator up and normally the amount of the air out of the vents decreases a bit.