if the car is idling (like in traffic) its not building much heat in comparison to driving at freeway speeds.
conversely, while idling in traffic there is little airflow through the radiator while there is plenty at freeway speeds.

i surmise that there must be some kind of blockage thats preventing air from flowing freely through your radiator at high speed, and that the coolant capacity is enough to cool the engine when the engine isnt producing much heat. leaves, insects, other debris could be in the fins of the radiator or the AC condensor, preventing airflow. i say check there first. clean out as nec, and observe for any changes in temps.

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'O=00=O'
bmw 2002.
long live the legend