how quickly does it get hot? and how hot are we talking?
the broblem could be one or two of a handfull, such has the headgasket or the water pump, etc...
have you got water in the tank?
if so what colour?
either way what ever the problem may be it would appear to me to be something you should get a garage involved with as with out being there to actualy look at the car i cant think of any thing obvious and audi's are not my speciality, sorry.