I've recently bought a 1997 Audi S8. The car has 109k on it and an excellent service history. In fact, it's never gone more than 6k miles between oil services. There is also evidence of two gearbox services (these are apparently a known problem area) and I've had the ATF and filter changed as well.

It's just gone through an MOT where the emissions numbers were brilliant. However, I am experiencing a juddering when at higher speeds (40mph+) that ranges from very subtle to very noticeable. At its worst it's like driving over a cattle grid. It is not a transitory thing but goes on for minutes at a time. It does not seem to be RPM or gear related. When travelling at circa 60mph I've tried changing down to 4th and 3rd and it continues. It seems to happen more during warm up, but not exclusively. Any thoughts?