you have popups in there and clicking on a forumthread opened a new window. Both big DON'Ts in my book. What I saw before closing your site in disgust was that the adsense ads do blend nicely, so those visitors who don't mind popups are likely to also click on your ads.
You shouldn't have pop-ups in a forum. Visitors tend to click and browse the forum for a long time. Popups can then become really irritating and you eventually lost your visitors.
I just clicked on two forums and the popups were really annoying. Actually after I clicked one I was annoyed.