Submenu submenu DotNetNuke solpartmenu

I have created a skin that uses solpartmenu. On a portal that uses a submenu, I get different behavior across browsers: in IE and Firefox, the submenus drop out just below the parent elements, as you'd expect. However, in Safari and Chrome, they appear at the far left of the page. I didn't think I was doing something too weird with my CSS, but I seem to be missing something. An example is here: http://www.codeswell.com/tpabuilders . I have added solpartmenu styles below. I can add the complete skin / CSS code if needed.

Can anyone shed some light on this?

/* SolPartMenu styles */

.MainMenu_MenuItem 
{
    cursor: pointer;
    background-color: White;
    color: #006309;
    font-family: Times New Roman;
    font-size: 14px;
    font-weight: bold;
}

.MainMenu_RootMenuItemActive
{
    text-decoration: underline;
}

.MainMenu_RootMenuArrow 
{
    display: none; 
}

.MainMenu_MenuIcon
{
    width: 0px; 
    height: 0px;
}

.MainMenu_MenuArrow 
{
    display: none; 
}

.MainMenu_MenuItemSel
{
    cursor: pointer;
    background: White;
    color: Maroon;
    font-family: Times New Roman;
    font-size: 14px;
    font-weight: bold;
}