Rough transition from one Preview mode to another

I’m using RW 6.3.1. Building a website using Stacks and Foundation. I find there is a very rough transition from Full Page preview into the smaller modes, but particularly into the iPhone preview. It would be great if, when you left full page and clicked on iPhone in Preview, the page would come up in the same place. But I find that it generally doesn’t. Another problem with Preview is that, when scrolling up and down, I get a lot of hangups and beachballs. Often, I can solve this by moving my cursor over to my desktop, clicking the mouse button and moving the cursor up and down once or twice. For some reason, that can work. But not always; sometimes I just have to wait.

If I do command/P and pull up a Safari page to look at what I have, then that works very smoothly; it’s just the RW preview that has the problem. But I like the way the RW preview gets more narrow for iPhone view than I can get with the Safari page.