if the ride.. ie body of the car feels lumpy at 80mph the rear wheels will need balancing before you even investigate anything..

On the A4 you need to balance the rear wheels otherwise you get a virbation, lumpy ride..

A lot of tyre fitters dont balance the rear wheels on cars as it does not make much differance.. however on the VAG stuff it does..

Do that first... it will only cost you a tenner at the most.. and at least you have ruled out an common issue..

If engine related.. please advise in more detail.

RicKT