Just got a call from the bike shop they replaced the smallest cog on the back wheel with the same (Advent Cassette COG). Same thing chain skipping. Then they replaced it with a different brand smallest cog (which is silver as opposed to the rest being black). They said the skipping stopped. I will be picking it up this weekend. Lesson learned stay away from Advent Micro Shift on these E-Bikes, problems right out of the store. Also surprised this bike came with a KMC Z9 chain instead of a KMC E9 chain which is made for E-Bikes.