I'm occasionally having the same issue. I'm using Chrome and Windows 7 Enterprise. There is a youtube video for an ad that pops up, starts playing and covers up the first post. It happened just now and I saw the ad was for Prosaro, but it's not every Prosaro ad. If you close the ad it pops back up immediately the instant you move the mouse.
The best thing I found to do is to close the ad and read the first post without moving my mouse. Then when I scroll down it pops back up, but it only covers the first ad. I also mute my PC, so I don't hear it.