sounds like the lockring, make sure that your cog is all the way tightened before you tighten the lockring. If the moving keeps up, the cog should move so easily that you can move it by hand. A cog this loose can lead to stripped threads.

So get out that chain whip or use the rotaflex method and then crank on the lockring with a lockring tool (a screwdriver can only do so much).