I had the same symtoms while towing my trailer tent on the motorway, temp light came on temp gauge went to full... undid the water filler cap and it was under pressure blew a load of air out(steam everywhere lol), luckily i had some water i slowly poured some in after letting it cool for about half an hour and it took about 2 litres so boiled quite a bit... cut a longer story short (mechanics humming and arring etc) it was the head gasket got it sorted but wasn`t cheap and its been fine since... thanks for the reply did you have the head skimmed or new head and how much did it cost you all in. thanks