This is something I noticed in the plain sandbox, well before I did any customization to it.
The right and left nav bars float as you resize the window, but don't have any minimum sizes. So, on very small windows, the text is covered by one, or both, of the sidebars.
You can test it here:
http://membracid.wordpress.com
happens in both IE and Firefox.
I'm suspecting this is a simple, known fix. Thanks!