Hi Paul. Welcome to the world of Flavias! The first thing I would be looking at is, is the water pump circulating the water fast enough, I had a car years ago that did the same sort of thing, turned out that the impeller in the pump was worn away, and could cope at low revs but not at high revs. I presume that their is not a build up of pressure in the radiator. Any water loss? Themostat opening and closing as it should? Or it could be something very simple like the sender unit in the rad is faulty, i also had a car that the voltage stabiliser was faulty, and all the gauges did not read properly, does the fuel guage read ok. Also does the guage go up not down when filled,i have seen cars which have had the polarity reversed, and people have not swapped all the wires on gauges over , so as you go faster the temp
gauge goes up , but with airflow through the radiator it should go down, and also check if rad clogged up inside and outside. Thats a few things to be getting on with. I am sure there will be more replys soon. Let us all know how you get on. Roger.