Chain and sprocket histories matter. Is this a new chain with a new sprocket? If the chain has seen use with a FW or cassette, it can be "stretched" and now longer run smoothly on a new cog. A new chain on a used cog can also give you issues.
Also, are your sprocket and cog compatible widths? You can run a wider chain on a narrow cog but not a narrower chain on a wide cog.
Ben