I used to have that problem with Opera. It's good to see people using something other than Internet Exploder. In my case, when the ad would shuffle on top of something, was when I scrolled down too quickly, or reloaded the page when I was already scrolled down looking at something.
Here's a tip. Go to the top of the page when reloading. Also, wait until the page is fully loaded before scrolling down. I believe the ad "knew" where to go in relation to your browser window, but if you scrolled down too quickly, it would end up where it was supposed to be... or so it thought. Where the ad appeared on top and where that location was when I scrolled were about the same place.
I have Opera 8 and I want to go back to Opera 7. Does anyone have that install file?