Blogger Support to the rescue

Steve from the Blogger Support Team sent his response to my email:

On occasion, the side-bar will shift when the content contained within the side-menu or a post extends the column beyond its designated width. We recommend reviewing the content in both columns to identify items that may be causing column expansion. In most cases, long links and wide images are the cause of the problem.

I am going through and removing links and pictures from the posts to see what happens. If that doesn't work I'll work on the sidebar links. Cross your fingers and toes.