Sounds like something that happened to me when I first got a track bike. The cog probably wasnt tightened all the way. The lockring was tightened as far as it would go against the cog. So there was some tightening and loosening going on with the cog. What you should do- Get a lockring tool. Ride really hard for a few minutes, and stop using only the brake. Tighten the lock ring as hard as you can- remember its reverse threadded. Repeat one more time just to be certain. Thats how I did it.