wait a minute...does it immediately go up and down with speed (or RPM)? that could be purely electrical.

assuming its not electrical, what is the ambient outside temperature when this is happening? this is similar to the behavior I experience on my 300D with my radiator blocked after about 60 degrees F ambient temps (I block the radiator in the winter to keep coolant hot). if that is the case, your thermostat isn't opening fully, and as others have suggested it needs replacing.

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'85 300D, 'Lance',250k, ... winter beater (100k on franken-Frybrid 3 Valve Kit)
'82 300D, 'Tex', 228k body / 170k engine ... summer car
'83 300TD Cali Wagon 210k, wife's car