Hi @Corine, this might be happening, because the whole page has not loaded before the Overlays are shown. I would perhaps consider add a page loader to your site:
This way, you can setup a loader screen to wait until the page has loaded, and then show the content. I common culprit as to why, is size of images, or a lot of different requests on the page, that have not finished loading yet.
Let me know if this helps !