For the high idle I would start off by checking for vacuum leaks. On the M103 the idle air control valve hoses tend to get brittle and crack with age. You could also remove the idle air control valve to clean and inspect it. It could very well be sticking open. As far as the overheating you can start off by checking the coolant condition, level and for any leaks. Since the car runs okay when moving I would suspect possibly a bad radiator or water pump, especially if they are original. Do you know if the car has factory or aftermarket (generic) coolant?

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1989 300CE