Can you clarify the "skipping"? Do you mean that the chain is riding up on the teeth of the cog, like it wants to shift, and then slotting back in to place? Or do you mean that you need to click up/down on the shifter twice before it'll shift? Is it only when shifting in one direction (up the cassette or down the cassette)? Does this happen on anywhere and everywhere on the cassette or only in a certain range of cogs?