So I felt the cog slip again, just a few hours after installing the new DA cog and Formula lockring. I was applying some pretty severe backpressure (skidding), and I felt the cog slip backwards just a little bit, until I heard it SQUEAK. After the squeak, it seemed as if the cog wasn't going to move anymore. I picked up the bike and brought it inside.

This is really beginning to get on my nerves. I followed the same installation procedure as before. Any other ideas?