Gap between menubar and banner

I am designing a website using ASP.Net 2.0 and C#2005. I have used a stylesheet for the background and the horizontal menu bar. I am dsplaying 4 menu items horizontally and below that I am displaying a Flash (.swf) banner. The page is being correctly rendered & displayed in IE8, Opera, FireFox, Chrome and Safari. But in IE 6 I am getting following problems:

1. There is a gap of around 25 px between the horizontal menu and the banner.
2. In the gap, the last 2 characters of the the last menu item description is getting displayed.
3. If I increase the menu items to 5 or more, the menu bar is being broken to 2 lines.

My Stylesheet is as follows :

body {
margin: 0;
padding: 0;
background-color: #329900;
background-image: url(Images/BG1.jpg);
background-repeat: repeat-x;
font-family: "Trebuchet MS", Verdana, Arial, sans-serif;
font-size: 12px;
}

a.nav, a.nav:link, a.nav:visited {display:block; width:140px; height:25px; background:#dca; border:1px solid #000;
margin-top:2px; text-align:center; text-decoration:none; font-family:verdana, arial, sans-serif;
font-size:12px; color:#000; line-height:25px; overflow:hidden; float:left;}

a.nav:hover {color:#fff; background:#764;}

a.nav:active {color:#fff; background:#c00;}

My Aspx page is coded as follows:
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>