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 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.