Problem with backgound-position-y

[carlosnpz]

The website I'm working on has a set of divs using stellar.js background parallax solution. From the 3rd on, the browser calculates the "background-position-y" value incorrectly, sometimes.

The images get a white space on top, as if the background had a margin. It doesn't happen all the time, but if i keep refreshing the page, this will fatally happen (on Chrome and old versions of Firefox).

Is there a way to solve this problem?