Yup sounds like your clutch is slipping. Time for a new one.

Are you chipped? If so try and go to one that is rated for more HP and TQ than a stock one. Thats prolly a good idea anyways. I think the ones for a Audi TT 225 will fit yours but I could be wrong as its been a while since I had a 1.8T but that should be able to hold up better than the original one.

Lee