"cassette wobble" When does it happen? if you have a repair stand can you see if the cassette wobbles only while you are coasting or also while when you are pedaling. Take the wheel off the bike and examine the cassette. Try to see if the cassette cogs move side to side when you push on them. If they do it may be that your setup lacks a spacer to snug them up. We don't know this because we don't know any other details of your drivetrain(we know about the wheels but nothing else). Are the cassette cogs moving when you try to wiggle them? If so, you may need to add a spacer to your free hub body. This is all that I have as a suggestion