Bouncing Brakes??

[baz-r]

most bikes come with a thin fiber washer between the wheel and discs on pritty much all the bikes i have had. you havent lost or dobbled up on any of the holes by mistake? or fitted it with the wrong rotation (should have arrow if so) i always try to refit my disc in exactly the same alignment it was fitted to the wheel if im reuseing an old one
your problem does point to disc warping by the sound of it