I had a look around the links that you provided, and I cannot see anything in particular that could be wrong ..
So here's a list of things that you might have to check to mail down the issue:
1. Check your widths in the CSS and make sure none of the posts are making any trouble.
2. Check for un-closed containers in the (X)HTML code ..
3. And perhaps you might want to validate your document a little better, that would help a lot in seeing what might be going wrong. You have a whopping 432 errors in your XHTML Strict DOCTYPE validation .. :D
If none of that helps, then post back ..