My 1988 300SEL has a heater problem. It will intermittently work at the MAX setting. When the car reaches operating temperature and the Climate control is in the MAX position the heater works sometimes. When the dial is then moved into the white normal operating zone it blows cold air. When I move the dial back to MAX it still blows cold air. I can then turn the engine off, immediately restart it and the heater will again work on the MAX setting. What can be the problem here. Any help is greatly appreciated. Thanks in advance.