I think you answered your own question. As you probably know by now, these cars are very sensitive to wheel/tire balance. It sounds like you know that the E39 wheels do not have the correct hub size for the E34 hub, so you need to find a hubcentric ring. Try one of the big tire/wheel places- Discount tire, etc- or I've heard that Tire Rack sells them too- I just don't have the details on that one.
I would suspect that the ring will help, if not cure, your problem. Let us know.
Bret.