MY VW GOLF IS OVER HEATING ON THE MOTORWAY NOT WHEN IDOL?

MY VW GOLF IS OVER HEATING WHEN DROVE FOR HALF HOUR NOT WHEN IDOL BUT THERES NO HEAT COMING FROM THE FANS. IVE CHANGED THERMOSTAT NOT THAT WHEN ON THE MOTORWAY WARNING LIGHTS SAY STOP AND THE WATER COOLANT LIGHT COMES ON WHEN I CHECK FILLER BOTTLE THERES WATER IN THERE VERY HOT
VW GOLF 1.6 SE 1999