I have been having this same problem with my new aerospoke track hub. I put into the aerospoke I have been using as a front wheel. I know exactly the problem you are talking about and I have had several mechanics work on it and think they have gotten it tight...but it hasn't wored so far and I have redone it myself a few times too.
Today I had a spacer put inbetween the cog and lockring cause there is a small area between the two thread sets that is smooth. It didn't really fix it.
The problem is that you get he lockring tight and when you try and get it the last little bit, it pulls the cog with it. this loosens the cog and tightens the lockring. when you pedal forward, the cog slips tight and the lockring moves with it, getting a little loose. I have tried using the cog tightening technique using the frame...and whilei am holding hte cog in place like that, I have a friend wrench on the lockring.
I have never had this problem before and I can't figure out how to fix it yet.
Let me know if you solve it.