It definetly isn't an ad, AdBlock would just block it. Even AdBlock-users like me get these redirects.
Also the random appearance is a sign it's not client-based but more of a host-based problem. What I discovered is that at least for me, I only get redirects when clicking on "forum-generated-standard-links", like "Unread posts" ( /index.php?action=unread) or stuff like that, never to specific posts (/index.php/topic,47.0/topicseen.html). I can't really verify this, but at least for me, this is true for the few times I got a redirect.
Interesting would be, what the redirects looks like in surf history, is it only a "direct redirect" or is the redirect across some sort of "spreader"-url which then redirects again to adultfriendfinder and others depending on user location.