Have this car since new. A few years ago I replaced a noisy air mix motor and also replaced the control wire and water control valve. Everything worked fine. This past winter I've noticed that only when driving at expressway speeds for about twenty minutes the air coming from the vents starts coming out cool. Usually keep it set at 68 but then have to bump it up about ten degrees for warm air. As I said this just started recently. Any ideas as to why this could happen?