That's a weird one. Does it misbehave on both sides, or only the driver's side? Usually sensor problems won't cause a transition at a middle temperature, it will be more like blowing warm until you set it for 60~61. But if the mix doors weren't working you'd get little or no change at one end of the temperature scale. If it's only the driver's side that misbehaves, it could be a mix door servo with a bad position pot. If it's both sides then that's a new one. Maybe a bad control head, because sensors don't behave that way.