All of a sudden the defrost/heater isn't working on my 78 280Z. It blows warm/hot air only in city driving(less than 50mph)at highway driving (more than 55mph)it blows in outside air or cold air. My A/C compressor belt is removed. All vacumm lines are connected. No major work has been done in relation to this system. Water temp is at normal operating range. I am puzzled!!! Thanks in advance.