Hot air!!

[A6 2.8 Quat]

Can anyone please help me......
I have a Audi 2.8 on a 99 bought this summer, and the hot air blower is cold. When your on a motorway it seens to get hot but as soon as you stop at a junction or lights it goes cold again. The cc air con seems to be fine and gets ice cold but now the winter is setting in i went to use the heat and found this problem, any help would be greatful,
Thanks.