Greetings and thank you for your post. Well it sounds like you may have a bad fan clutch. In some cases you can check the fan clutch for looseness by grabbing the fan and using a push/pull motion to check for excess play. If the vehicle temp is fine at idle it would certainly seem like the fan clutch is working but if the fan clutch is not able to keep up with the load that is being caused at highway speeds due to excess play leading to a slip like condition then it would cause an overheat condition at higher rpms. One more thing, I would double check the radiator to make sure there is no restriction by getting the vehicle to operating temp and running your hand across the whole radiator making sure the temp feels the same all the way across.If you find any cool spots then I would definitely suspect a restriction in the radiator new or not I have encountered this as well.