I can almost guarantee your cog is too narrow. I've been having this problem lately with Soma cogs and IRO hubs / ProMax hubs. I tried using a little spacer. It worked on the IRO hub, but on the Pro Max, the first time I skidded hard I destroyed the spacer. I switched to a Dura Ace cog and the problem went away. I avoid Soma cogs at all costs now. Does anyone know if the Surly cogs are the same width as Dura Ace?