How to stop header jumping ?

Any screenshot on what you meant by jumping?

 

This is how Divi works for injected elements. It will first load the content, then only the CSS to style it.

There are 2 workarounds:

1. Use a preloader plugin to hide the page until it's fully loaded. There are plenty free preloader plugin out there. For example: https://wordpress.org/plugins/the-preloader/

2. Go to Divi > Theme Options to set up the below settings. After you set that, visit every page of your site & Divi will save a copy of the CSS of each page in file format. The next visit to your page will have the CSS file load first & you don't have to see the 'flashing'.



But from what I see, you already enabled the Static CSS File Generation. One of the issues with Static CSS File Generation is that it doesn't work with the archive / category page, but it should work on the rest of the pages. You just need to visit all your pages once & it will generate the CSS file to avoid seeing the 'flashing'.

 

Not everyday. Once any visitor view that particular page, Divi will generate a CSS File, so that for subsequent visit to that page will be served the CSS File to avoid seeing the flashing. The CSS File will be kept on your server until you made some changes to your site / page.

So far, I'm aware of these 2 solutions only: Static CSS File & preloader.