Heater stops blowing hot air -water temp gauge drops

The water temp gauge rises as normal and the heater blows out hot air when the car idles and the cooling fan does comes on ok, but when I am driving the car the gauge drops right back to the left hand side and the hot air from the heater cools down……does anyone know whats up. The car is a 1982 1.8Gti Convertible.
Cheers

You should look at fitting a new coolant thermostat .

  

I will try that and see how it goes. Thanks for the advice.