Check your coolant, I'll bet it's low. This will cause circulation failure in the heater core at idle and will also cause poor circulation through the radiator. The head will get hot due to lack of coolant circulating, but the thermostat won't open becuase the coolant isn't moving past it. When it does heat up and open, a slug of cold coolant from the rad will blow through, etc.

A bad thermostat will have no effect on the heater circulation.

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!