Aux fan should be on and the visco clutch locked up at that temp. I'd replace the visco clutch and find out why the aux fan won't run before getting too excited.
My 300D runs 100+ in slow traffic with the AC on, never had a problem yet.
As stated above, cracked head will show cold coolant tank pressure, oil in the coolant tank, water in the oil, or "mystery" coolant loss.
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!