This video discusses and demonstrates what most freewheel wobble is caused by. While most people will point to what they see as the cause - the freewheel - most freewheel wobble is caused by machining defects in the hub shell of the hub that the freewheel is mounted on. Basically any "deviant" motion caused by the hub shell machining defects are inherited by the freewheel.
www.mrrabbit.net