Try turning on 'use smooth scrolling' in System Preferences->Appearance
The screen refreshes itself a certain number of times a second - smooth scrolling ensures that the Mac does all the scrolling work in between each refresh and makes the scroll look perfect. Turning smooth scrolling off can mean that the screen updates mid-scroll which you can see as a 'tearing' effect in the page.
Turning smooth scrolling off is quicker and more efficient, and the effect isn't a big deal. It's good that Apple gives a choice.
Let us know if that fixed your problem.