OK, well the chain is very new, but here's my observation when backpeddling- If I do it slowly enough, the cog on the lower pulley does not spin until I pull it far enough and it finally spins. So it seems like I may just do a slight backpedal while riding and this happens. But I lubed the heck out of that cog to no avail. I could try loosening it, or... replacing it?