I would check the aux fan operation and visco clutch first -- likely the visco clutch is bad (assuming you have one, I'm not sure). Temps hanging around 100 C in traffic sounds a whole lot like a bad visco clutch -- both of my cars did that before I replaced them.

A low temp thermostat won't help.

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!