I have experienced this with many wordpress sites using smooth background scrolling in Internet Explorer, but I haven’t found a solution for sydney theme yet, allthough flipping through similar questions here and trying a hell of a lot CSS and JS tweaks.
So the problem is always when using a fixed background, e.g. in a standard text section. When scrolling with the browser scrollbar, it’s fine, but when scrolling with the mouse wheel, the background jerks back and forth.
I haven’t my page online yet, but I saw that a page of another user, having a similar problem still has this same issue: http://staffordshiresurfacing.co.uk/ (in the lower part “what our customers say”).
So, which CSS element can be modified to switch for example to a scrolling image like in smaller devices for IE only?