Was thinking that's what happened FBinNY. When I started my ride I felt the rear wheel stutter/slip a few times, and when I'm on the trail I end up releasing the tension of my legs to slow down a bit before I use the hand brake and see if the left hand lane is clear before passing; I think this was causing the rear cog to loosen up a bit and when I stood up in the drops and started to sprint past someone the cog tightened up again caused a skip; or jumped off and back on making the loud popping noise, and caused me to eat pavement. The play in the cranks probably worsened the problem as well. I ride the fixed only on trails when I want to do sprinting intervals. The hub is stepped threaded as well.