Zen 5.x single right sidebar floats under content with MSIE 7 and 8

I'm using a responsive theme under Zen 7,x 5,5. It has a single right sidebar. It works great under Windows Firefox, Opera, Chrome, and MSIE 9. Under MSIE 7 and 8, the sidebar floats to below the main content. This occurs with wide browser widths where there should be room for the sidebar.

I have researched and struggled with this, and can't find a solution. The subtheme files are pretty much unchanged from starterkit files.

This appears to be the code in responsive.css used on the pages:

----------------------
  /**  The layout when there is only one sidebar, the right one.  */

  /* Span 2 columns, starting in 1st column from left. */
  .sidebar-second #content {
    float: left;
    width: 66.66667%;
    margin-left: 0%;
    margin-right: -66.66667%;
  }

  /* Span 1 column, starting in 3rd column from left. */
  .sidebar-second .region-sidebar-second {
    float: left;
    width: 33.33333%;
    margin-left: 66.66667%;
    margin-right: -100%;
  }
----------------------

Decreasing #content width to 60% didn't help.

I applied float:right to .region-sidebar-second. The sidebar floated to the right, but of course it is out of position. Plus, it didn't float to below content with a narrow browser window. So I don't think that is the solution.

This may be a MSIE bug or even a Zen issue, but I am surprised no one else has encountered it.

I would appreciate any help and advice - I'm stuck on this one.