As pointed out already, less-than-perfect rotors could be the root cause.
How are your wheel bearings? Just a bit too loose can cause this sort of problem.
My experience with OEM rotors thus far has not been good. Many have showed up bad from the get-go.
After going through 3 sets the last time I changed them, I gave up and had a local machine shop turn them.
Some here believe that turning MB rotors is a no-no. MBDoc posted a good ways back that this is indeed a legitimate procedure. After the rotor turning, life was good and I was no longer spending week-ends replacing faulty rotors.
For all the anti-rotor-cutting types, look at what M.B.DOC had to say about it in this thread: