Navbar & Content jump when scrolling down. How to fix?

My website at hollis-tv.com is jumping when you scroll down from the top. I cannot figure out why it is doing this. I believe it is something built into the theme we are using but we aren’t getting any response from the theme developer on the issue. I have tried combing through the coding and nothing is jumping out at me.

(The theme is ceskalipa in case it matters)

I have not altered the coding or the css other than a few minor fixes which were necessary – and were added to the css on “add-on” mode. But I also tried resetting it (just in case) and the jumping problem still occurs.

Please does anyone know what is causing this?? It makes the website look glitchy and I want to fix it.