Issues with subnavigation menus changing positions when using CSS in IE7

Question

The subnavigation menus (located just below the blue tabbed navigation) are showing up in a different position in IE7--they display correctly in Firefox and IE8:

https://www.diservio.com/newsite/vehicle/auto-insurance.html

I created a separate IE7 CSS file and added conditional comments in the HTML. Here are the properties I modified:

#topnav {padding-bottom: 10px;}
#subnavbg h3 {margin: -663px 0 0 -340px;}
#subnavmenu ul {margin: -663px 0 0 -340px;}
#leftsubnav {MARGIN: -601px 0 0 -550px;}

Here's a portion of what the HTML looks like--all of the divs are nested inside the main div #container (not displayed):

 <div id="subplacement">
        <div id="maincontent">          
 ...        
                <h1>Auto Insurance</h1>
    <p>At Tony DiServio Insurance we know how important it is for you to protect yourself and your loved ones when you get behind the wheel.</p>

        </div>
    </div>
 ...
 <div id="subnavmenu">
        <ul>
            <li><a href="auto-insurance.html" id="autolink" title="Auto Insurance Link"><span>Auto</span></a></li>
            <li><a href="motorcycle-insurance.html" id="cyclelink" title="Motorcycle Insurance Link"><span>Motorcycle</span></a></li>
            <li><a href="boat-insurance.html" id="boatlink" title="Boat Insurance Link"><span>Boat</span></a></li>
        </ul>   
 </div>
 <div id="leftsubnav">
            <ul>
            <li><a href="auto-coverage.html" id="coverlink" title="Coverage Link"><span>Coverage</span></a></li>
        </ul>
        </div>
 </div>

The submenus are also bounced into different positions on the Home > Vehicle page and Vehicle > Auto Insurance > Auto Coverage page. I can't figure out why. Any suggestions on what I need to fix for IE7?