High temp at highway speed is indication of inefficent radiator cooling ..
Separate the radiator from the condensor on top and see if you have lots of dirt/blockage in between .. a good cleaning with a garden hose works here.. then retry..

also , check all 4 wheels after a run for heat [ dragging brake possible]

also check coolant a/f mix %. You do not want more than 50/50

100C at high ambient is not out of spec, but a thermo that does not fully open will also do this .