Hey @Volkmar_Sitte,
As shown in the first method in the article I shared above, go ahead and try uninstalling the Realtek driver from Control Panel > Uninstall a Program. Once you’ve done this is there any change?
I’d also like to recommend changing the drivers you use here so we can see if there is any improvement. Take a look at this article about using Microsoft HD Drivers. When using different drivers is there any improvement?
Thanks!