Sorry to tell you this but no one seems to reply to posts on here any more especially ones to do with IE and the dropping sidebar.
I spent a long time trying to figure out the same thing and finally got an answer from Guido at http://www.easywebtutorials.com/
The only answer seems to be use a table……. see the links below.
easywebtutorials have some great layouts that are easy to modify using a basic table an css.
1. Searching in the forum for “IE sidebar down” – it gives you 234 hits. Do you think we need a 235th topic about the same?
2. The answer for fixing it is always the same basic idea: you have something (an image, a code line, a link etc.) either in the sidebar or in the content area that is WIDER than the width of its container element.
3. Which means you don’t really need to use tables to have a site that looks decent cross-browser…