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Look for a soft heater hose -- suction (return) side is most likely. Mushy hoses can collapse, preventing coolant circulation through the heater core -- this is most likely to be the problem (poor circulation).
You should be able to feel the aux pump running at idle by the internal vibration. It its not running, you can get exactly the symptoms you are describing -- nice heat at idle, but none at speed.
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!
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