Does the lock ring fit all the way in against the cog's set of threads? And where are your spacers ... between the hub shell and the cog, or between the cog and the lock ring? I have never needed spacers ... I have always been able to get the cog snug up against the hub shell (have never used a Surly cog, though), and I've always been able to get the lock ring snug up against the cog. That way, if the cog wants to try to spin off, it inevitably tightens the lock ring. It seems impossible that the lock ring could spin off against the threads without stripping them.