Another thing that can cause this (along with large temperature swings) is a dead foam hose to the aspriator motor for the interior temp sensor. It's behind the glove box. Replace with something like pipe insulation or heater hose and make sure the little blower motor is working.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!