Are you doing parallax scroll by turning scroll off of the rectangle, or background?
DON'T change the scroll effects of the rectangle itself, by clicking the scroll effects tab while the rectangle is selected.
DO this: select the rectangle with the marker in it>open background color/image box at top left by shift clicking the arrow>set the background image, then click "scroll" tab>change everything to 0 in here.
To me it isn't shaking (google chrome), sometimes the shaking occurs in browsers because they use different kinds of animation.
When i get to a certain point however, i can see the image suddenly jump up (as i almost finish the scroll).
which leads me to believe you turned scroll off for the rectangle, not to background.
Hope that helps!