Visco clutch, probably.
Also, check to make sure you have the engine full of coolant by squeezing the upper radiator hose (engine cool, tank cap off), the pinching the overflow hose shut. Let go of the upper hose, then repeat. If air burps out, keep repeating and adding coolant til only coolant moves into the tank when you squeese the upper rad hose.
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!