Your beginning vertical scrolls are also slightly jerky, just not as noticeable as the horizontal scrolls. I have repeadedly had problems with the vertical scroller but have been able to get smooth horizontal scrolls. What are the specs on the computer and hard drive that you are using?
I recently did some serious scrolling of photos across the screen and they were not smooth, no matter what I did and what I had my preview set at. However when I rendered it out and played it on a DVD player it was smooth as silk? Who knows?