Check that the crown race isnt loose on the steerer tube.They are normally quite bombproof- thats the only thing I can think of.What I do is nip the centre bolt down, then the stem bolts, then centre bolt and finally tighten stem bolts.Thatway its all clamped gradually.Make sure that the top of the forks is a few mm down inside the stem and the top bolt doesnt foul forks.