I know you don't want to hear this .......... But, I've had RD's that just would not shift properly and, no amount of adjustment would correct it. Most RD's are mass produced items that have several parts that are stamped & pressed together and, sometimes they're out of tolerance. Consider upgrading you RD to a higher model level. None of Shimano's RD's are any good until you get to the 105, Deore, or maybe Sora level stuff.