If the heating system helps to cool the car, then the rest of the cooling system is not doing its job. In your case I suspect either the radiator or the thermostat, especially if the thermostat is new. There was a bad production run of certain brand(s) and you need to research here to find a good one and try it first, that's the easiest thing to start with.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles