they dont get enough air on the radiator
you will notice that your problem becomes worse after a highway cruise and then slow/stop and go traffic, the temp guage keeps getting higher with slower speed
with a 50 to 1 air to coolant ratio, there is not enough air passing through the radiator core to transmit the heat.
shroud the radiator fan and increase the fan blade count, also , make sure your lower radiator hose is not collapsing under suction load
hope this helps