The fact it dosn't overheat in town or idling eliminates fan and shroud issue. The fan and shroud are for low speed cooling only and serve no purpose at highway speeds where airflow thru the radiator is rammed thru by the very motion of the vehicle thru the air.
Heat buildup due to low airflow thru the engine compartment, is also not likely to be the issue, as again at highway speed there is plenty of air flow to prevent that from occuring.
Redfastback may be onto something, you are running a thermostsat right? No thermostat at all would allow the coolant to flow thru the radiator too fast to allow it to give up heat and too fast thru the engine to absorb heat, at highway speeds allowing heat to build up but would be fine at low speed. Good stat is a must.
I personaly agree with rklein383, sounds like the lower hose is collapsing at speed. Find and install a lower hose spring and you should be good.
