Cog wobble is normally associated with loose freewheel bearings, poor threading on the hub or freewheel body or loose or damaged cogs. If all the cogs appear to wobble, it is not the source of your problem. If only the small cog wobbles, it probably is the problem. if the cog is loose it sounds like it is missing a spacer or has the incorrect spcer. What is the make and model of the freewheel?