This happens in Chrome and Opera and does not happen in Firefox. When some object is moved down by Jarallax at similar speed with scrollbar the animation for this object is jerking.
So, when you scroll down it affects any object which has a rule like:
jarallax.addAnimation('.some-object',
[{progress:'0%', top: '0px'},
{progress:'10%', top: '100px'},
{progress:'100%', top: '100px'}]);
And everything is fine for objects moving in opposite direction:
jarallax.addAnimation('.some-object',
[{progress:'0%', top: '100px'},
{progress:'10%', top: '0px'},
{progress:'100%', top: '0px'}]);