The auxillary water pump may be bad, too -- this will restrict flow and leave you with no heater core circulation at some speeds. I'm not sure how to check it -- mainly it should be running at low speed, anyway, and you can feel it buzzing.
Note that if it is bad, it can fry the pushbutton control unit from excess current draw.
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!