WordPress WebSite is jumps while scrolling bottom [closed]

You have some css that is conflicting and causing the issue when the top menu sticks absolute on scroll down and the header hides.

In your media.css on line 340 you have:

    @media (min-width: 768px){
    .main-navigation {
        clear: both;
        display: block;
        float: left;
        width: 100%;
    }
}

Changing that in live dev tools to:

    @media (min-width: 768px){
    .main-navigation {
        clear: both;
        display: block;
        float: none !important;
        width: 100%;
    }
}

Seems to fix the issue and make it smooth again.