Mine did exactly that about two years ago when it was on 30,000 miles.
It overheated on the motorway, pulled over, called RAC, by the time the RAC had arrived it had cooled down, and they could not get it to overheat again.
The only difference is mine did it again on the way to the garage, it would only over heat under load.
Anyway, turned out to be the good old plastic water pump, was replaced for a metal one.