the disc is wallowed out.  it's not tight on the flat part of the drive cup.  You probably had a loose shoe which let the disc flop around causing it to open up the hole which was originally flat on two sides and round on the other two.  Now it's rounded all the way around and loose.  Easy fix.  Buy a new disc and replace the worn one.  Tighten up the shoes until they just barely touch the disc.  This will keep it from moving in and out.