Before you jump to the conclusion that it has something to do with your computer, try viewing the same websites on another computer, ideally at the same time you're trying to view them on your computer. You could also tell us the name of one or more sites where this happens so that we can look at it and see how it displays.
The advertising elements you observe to be missing may be coming from a separate web server from the web server used for the main website you're viewing. That/those server(s) may be experiencing problems causing downtime and that may be the crux of this; if so, it has nothing to do with your computer.