560sl heater won't stop
 

When I drove the car the other morning the heat should have come on but didn't. It was simply outside air. Later that day the heat did come on based on the climate control setting, but wouldn't shut off. I had to manualy shut it off!l I've had this happen to me before and checked the monovalve. It seemed fine. It seems that the climate control is fine with either all cooling or all heating, but doesn't do ok with switching from one to the other. It gets stuck on either hot or cold until something unsticks it. Does anyone know the most common reasons for this?

Thanks,

Steve