3rd column in 3-column layout breaks in IE 6

Hi all -

We have a standard three-column layout. We've tested it in numerous browsers on both Windows and Mac, and it looks great. It also XHTML and CSS validates.

The only browser that is misbehaving is IE 6. The majority of the pages look fine, but two in particular break. What breaks is the right-hand column, which displays further down on the page and on top of the second column's content:

http://www.urbanturf.com/about/
http://www.urbanturf.com/articles/category/neighborhoods/

I think this is the so-called Having Layout bug that my CSS book discusses, but I can't be sure because I don't have enough experience. I am relatively new to CSS and am working with code that was provided to us by a third-party designer.

I do know that our site uses the 960 grid system stylesheet (http://960.gs/)

Here is the XHTML code for the column that is causing problems:

<div id="tertiary" class="grid_3">
    <div id="marketplace">
        <h4 class="sifr">Marketplace</h4>
        <p>Homes, condos, and services in the DC area</p>
        <a href="..."><img src="..." alt="Marketplace" /></a>
        <span class="more">&raquo; <a href="...">More</a></span>
    </div>
    <div id="ad-tert">
        <!-- BEGIN - the 160x600 wide skyscraper -->
        (skyscraper JavaScript code)
        <!-- END - the 160x600 wide skyscraper -->
    </div>
</div>

I tried adding both "display: inline-block;" and "height: 1%;" to the grid_3 class styling in 960.css, but it had no effect, so I took them back out. (Those are workarounds to the Having Layout bug suggested by my CSS book.)

Can anyone help?

Thanks