Checkt he Rotor Adjustment on the rotor. should be an allen key. I had a similar problem, and when I changed pads, I noticed the rotor had a lot of play in it, tightened down the allen nut or bolt or whatever you want to call it, and the rotor had less play and my brakes feel great.
I believe this is some kind of adjustment tot he rotor to remove play as it wears.
I am not sure on this, but pop off the wheel and look for the allen bolt to tighten down if it can be.
Could be something in the suspension like a worn bushing, but I'd say its either that adjustment, or a dirty sensor..
Could be warped slightly, but I would think that would be felt at any speed.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.