On my '82 300SD I am having erratic operation of the climate control. I am sure that all the vacuum doors are operating, the problem is that often I get no heat/air out the floor vents just out the defroster. If I switch unit off and push the slow speed fan button and then switch back on to economy everything seems fine and will run that way till I push one of the other buttons and then I just get defrost air. IS the the push button control that is fritzing or is it something else? HELP!