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.