Vibration,coming and going

[devo]

hi all,im getting a funny vibration from my a4 avant.if i go over 60 mph,the whole car starts to vibrate and then it stops again in,say 5 second intervals.ive changed the front tyres as they were worn and thought they were the cause of the vibration.the wheels aren't damaged or buckled so they aren't to blame either.my missus's 04 a3 has the same problem but i put it down to her dmf being knackered.anyone else have the same problem or a solution.