i have the same thing...its the wheels not being centered on the hubs...mine started when I got them back from powdercoating...I put them on and I only get a vibration above 70mph and its a pulsating vibration...every 3 seconds it vibrates for a half second then stops, then 3 seconds later again, then stops...

when I put my rotas on the vibration does not come back. I ended up having to sand down the hub on the stock rims cause the powder built up and was not seating correctly on the hub. I think I may have sanded them down too much cause I still get the vibration every once in a while.

easy fix...get some new aftermarket wheels...lol

if you have a buddy with another S, borrow his rims for a day and drive around and see if it fixes it, it should

 

ya my wheels are powder coated bronze is there any way to fix it with the stock wheels on the car? i get the same vibration that you are saying word for word..