It's definitely related to the javascript, however there is enough other javascript on the page, it's hard to say if something else is causing it. What you probably need to do is setup a second site, pointing at the same database, with nothing running but the bare forum and see if it still persists.
This is actually a good idea for most everyone that does a lot of includes or portal boxes. That way it's easy to isolate it down to what might be causing the problem.
It's not a stock problem, so I'm assuming there is a conflict with one of the side boxes, or maybe some of the inserts that have javascript in them.