Elements in fixed position can be tricky with page sliding transitions.
To avoid flickering on transition when a page is scrolled, you need to make sure of two things :
UsePageSlider.js
PageSlider ensure that pages are always at 100% height so that scrolling don't produce jumps on page transitions.
In order to allow scrolling in your pages with PageSlider.js, you must allow overflow scrolling on your child.
Use data-fixed attribute
When the pages are sliding, children elements in fixed position don't follow their page. To prevent that, the trick is to set them in absolute positioning.
The data-fixed attribute works just like that : add data-fixed="absolute"
on your fixed elements and they will follow their parent.
You can find more informations in the PageSlider's Documentation