My old 260Z was doing the same thing. It ended up being a seized shut thermostat. The car was only being cooled though the byepass hose! It was fine at idle, running around town etc. but as soon as i got on the freeway it would get hot!

So your rad could be blocked. get it to operating temp and feel the top rad hose, then the bottom rad hose. If the bottom is cooler your rad is blocked! Or look at the rows inside theough the rad carp hole. is there any #### stuck in there, blocking them? Take the rad to a radiator shop for a flush out.

Its not the fan/s. Fans dont come on at FWY speeds, only when your going slower/stop starting. Which is when his car runs cooler.