mitch:
Could be that your right shoulder may be lifting on the BS, or in other words, your shoulder turn isn't flat enough. That dropping feeling might be a compensation as a result of a steep right shoulder turn. All you really need on the backswing is to fold the right arm, apply some extensor action, and let the body turn. Unless you are using a turning shoulder plane where you turn your body and lift your arms at the same time, you never consciously lift from the shoulders, nor do you ever lift your shoulders from the body. If you watch the sideview swing sequences of pros, all they really do is fold the right arm, apply extensor action and turn their bodies. Because they are extremely flexible, it appears they are doing a whole lot of lifting, but they aren't. Getting the hands to shoulder height is sufficient.
Chuck Evans' site had(has?) a short video clip demonstrating this with a piece of string. I don't know if its available since his site went pay.