What you describe sounds exactly like the problem I have on an 11 tooth small cog. If I switch cassettes to one with a 12 tooth small cog, no problem. So I left the cassette with the 12 tooth on. I couldn't figure it out either. This was on a 9 speed system. The problem was with a SRAM cassette, the Shimano cassette with the 12 tooth small cog does not have the problem.