Sidebar Overlapping On Content

At certain screen widths on my website, the sidebar overlaps on top of the post / article content. How do we keep that from happening, while still keeping the post text area at 750 px? Here is a link to a post page with the issue, and you can see the problem by maximizing the screen and then slowly making the screen smaller:

If it is helpful, I have granted support access via the wpmu plugin.

Thanks for your help!