Re '82 300SD. Climate control switches blow air at normal rate (auto and full-on), but it only warms up at idle (e.g. stopped at traffice light). Otherwise it is cool but not cold. I replaced the thermostat; engine warms up quickly to operating temp. When fan turned off (not on auto or full on), I can feel nice warm air at side vents, so warm air seems to be in the hoses. But press the buttons and it's only cool air. Is this a monovalve issue? Something else? Thanks.