Maybe the base of your cog is just a tiny wee bit too narrow, and the lockring isn't actually locking down on the cog enough. I had this problem with a Surly cog. I put a 1mm steel spacer between the cog and the hub. Otherwise my lockring would bottom out on its hub threading before putting enough force on the cog itself. Hopefully this is clear... Please try this before using any kind of loctite.