Well what I would advise you look at is the chain line coming down from the jockey pulley (top pulley) of the der. to see if it is in a straight line with the cogs. If not turn the barrel adjuster either clockwise or counterclockwise until it is. Then get on ride it and shift through the gears to see if that got it. If not go replace the chain. If this persists or is worse than go replace the cassette. That should fix it.