Mercedes: Overheating 1977 MB 300 D, fan clutch, temperature gauge

Question
While at idle the temperature gauge rises slowly all the way to redline.  Once the car is underway the temperature reading drops rapidly.  I've had the thermostat replaced (twice), a hole drilled in the thermostat, the water pump replaced and the radiator replaced.  The system has been pressure tested and everything seems OK.  The shop I go to has given up.  After all this work, the same sympton persists, although it takes much longer for the temperature to rise.  The car probably has around 250k miles on it, but runs good and seems to be in good condition.  It was losing water through the heater core, but that has been bipassed now.  Any insight?

Answer
I need your geographic location to recommend a GOOD MB shop.
You need immediate hands on help.

If the thermostat was installed backward = overheat.
If the fan clutch and/or aux fan circuit fail = overheat.
If the temp sensor is bad = false signal.

Please stop driving it, until this issue is fixed.