Vibration between 40 and 50 mph

Hi I don't know if any one has had similar problems on there car. When travelling at between 40 and 50 mph it gets quite a severe vibration not dissimilar to wheel wobble, (wheel balance has been checked). The car in question is a 1992 Audi 80 Quattro 2.8L. Any help or ideas would be appreciated. Many thanks, Mark