I don't think the radiator is that underdesigned. A properly functioning radiator and cooling system should not allow the coolant to reach 115C in the conditions that you describe. Is the electric cooling fan functioning properly?

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Ali Al-Chalabi

2001 CLK55
1999 Dodge Ram 2500 Cummins Diesel
2002 Harley-Davidson Fatboy
Merlin Extralight w/ Campy Record