it's water pump time my friend. Do the timing belt and tb tensioner while you're in there. The water pumps have plastic impellers and over time, the many heat cycles on the pump will degrade the impeller and when the car gets up to temp, the impeller expands faster that the pump shaft which is made of steel causing the impeller to freewheel in the shaft. i would use a water pump with a steel impeller.
2003 TT Coupe, Mauritius Blue exterior aniseed yellow and ebony interior.
Tiptronic 6 speed transmission.