I know its been ruled out already but the thermostate could get bloked shut on the freeway because you are getting enough air flow to the radiator to cool the car to the point that the thermo shuts but the surrounds of the thermo are still effected by the temp so in some way blocking it from opening. The water pump may also be an issue as it might be getting some free spin happening and not pushing the water around enough. I had the problem in my HSV and it would free spin temp would go up finnaly it would get some grip and it would drop from 3/4 to 1/4 temp in about 5 seconds
but in all honesty take it to a radiator shop so that you dont waste time and $$$ trying to diagnose it by the time you have checked/replaced all the trobel shooting items you would have probably spent more that taking it to an expert in the first place.

If its happening on the move then i would go with a radiator flush.
Heaps of imports have blocked radiators and at the very least it would be 50-100 dollars spent in peace-of-mind as the radiators are a little on the small side anyway.

Ben

I had a Skyline (R32) that would unexpectedly overheat - my problem was the thermostat would get stuck shut, then open suddenly and 20C water would meet 120C water - steam would piss out everywhere and the temp would go from C to H in about 2 sec flat....so if you don't have those symptoms and also the rad doesn't seem to be losing fluid - i'd bet on blocked rad too.

Some place will test them for nothing (if you pull it out of the car). Auto Cooling in Clayton did that for me - real good guys there.

My 2 bobs worth ....
wont be the clutch fan, as the problem would be reversed. Clutch fans are meant to pull at low revs and not at speed. I would look at the radiator. Working at lower speeds but when you push coolant through at higher velocity and temp - the radiator cant remove enough heat in the shorter time and higher temp. Flick the radiator - if not cured then you still have a new radiator Good luck!
Cheers

radiator is blocked or your clutch fan (if it has one?)
spins too easy at higher revs not pulling the air through

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.

GOOD news i've got the same problem with my 120Y try the water pump, it might look good but it might not be try pulling it apart and putting it back again. if new one needed try repco.

well it's not the cap either, changed that with no success (unfam you owe me $15, lol jokes)

must be blocked rad as said, only that or the pump left, but it keeps it's cool at low speed so must be blocked rad....

Blocked radiator.

had similar problem with the sedan, ended up being radiator cap... replaced that and have now found that when the radiator shop rewelded the top tank back on, 2 of the welds are a bit thin and are leaking under load (dont ask how i tested this but it did involve driving the car)

a slight mist would come out and the fans would disperse it through the engine bay.

If it still does this after they fix it on monday, its either the bottom hose collapsing or water pump is shot....

should be very similar on sr20det.

My solution to you, sell the 180 and get your $$$ back. then you can drive the mrs coupe for a daily.