VS 2010 On Client click is not running as expected

Dear All

I have weired situation i just wrote a simple java script function whichh will return true or false

even when the function returns false my server side button click event was firing.

this is not happening regullarly

i am using vs 2010 and ie 8 and vb.net 3.5

this is not happening all the time , please tell me your sugegstions

please check my below code

<%@ Page Language="VB" AutoEventWireup="false" CodeFile="ViewRateCard.aspx.vb" Inherits="ViewRateCard" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
.style1
{
width:15%;
}
.style2
{
width: 10%;

}
</style>
<script type="text/javascript">
function popNewRateWindow() {
var url;
var left = (screen.width / 2) - (400 / 2);
var top = (screen.height / 2) - (400 / 2);
url = 'http://localhost:6500/merabell/newRateCard.aspx';
newwindow = window.open(url, 'name', 'height=200,width=150,top=' + top + ', left=' + left);
//newwindow = window.open(url, 'name', 'height=700,width=800,top='+top+', left='+left);
if (window.focus) { newwindow.focus() }
document.getElementById('btnNew').disabled = true;
return false;
}
function validateDdlist() {
var validate = false;
var val;
var intext
val = document.getElementById('ddlRateCard').value;
if (val > -1) {
validate = true;
} else {

}

val = document.getElementById('ddlAdSize').value;
if (val > -1 && validate == true) {
validate = true;
} else {
validate = false;

}

val = document.getElementById('ddlAdColor').value;
if (val > -1 && validate == true) {
validate = true;
} else {
validate = false;

}

val = document.getElementById('ddlAdFreq').value;
if (val > -1 && validate == true) {
validate = true;
} else {
validate = false;

}

val = document.getElementById('ddlAdPos').value;
if (val > -1 && validate == true) {
validate = true;
} else {
validate = false;

}

val = document.getElementById('txtRate').value;
if (parseInt(val) > 0 && parseInt(val) != 'NaN' && validate == true) {
validate = true;
} else {
validate = false;

}

alert(validate);
if (validate == false) {

document.getElementById('divErr').innerHTML = "Please Fill The Mandatory Fields with Correct Values";
return validate;
}

}

</script>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="Scriptmanager1" runat="server">
</asp:ScriptManager>
<table width="100%" >
<tr>
<td colspan="6" align="center">
Test Project
</td>
</tr>
<tr>
<td class="style2" align="right">
<div id="div1" runat="server">
*</div>
</td>
<td class="style1" align="left">
Rate Card:
</td>
<td class="style1">
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlRateCard" AutoPostBack="true" runat="server">
<asp:ListItem Selected="True" Value="-1">Create or Select Rate Card</asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</td>
<td class="style2" align="right">
<div id="div2" runat="server">
*</div>
</td>
<td class="style1" align="left">
Ad Rate:
</td>
<td class="style1">
<asp:UpdatePanel ID="UpdatePanel2" runat="server">
<ContentTemplate>
<asp:TextBox ID="txtRate" runat="server"></asp:TextBox>
</ContentTemplate>
</asp:UpdatePanel>
</td>
</tr>
<tr>
<td class="style2" align="right">
<div id="div3" runat="server">
*</div>
</td>
<td align="left">
Ad Size:
</td>
<td align="left">
<asp:UpdatePanel ID="UpdatePanel3" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlAdSize" runat="server">
<asp:ListItem Selected="True">Select Ad Size</asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</td>
<td class="style2" align="right">
<div id="div4" runat="server">
*</div>
</td>
<td align="left">
Ad Color:
</td>
<td align="left">
<asp:UpdatePanel ID="UpdatePanel4" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlAdColor" runat="server">
<asp:ListItem Selected="True">Select Ad Color</asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</td>
</tr>
<tr>
<td class="style2" align="right">
<div id="div5" runat="server">
*</div>
</td>
<td align="left">
Ad Frequency:
</td>
<td align="left">
<asp:UpdatePanel ID="UpdatePanel5" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlAdFreq" runat="server">
<asp:ListItem Selected="True">Select Ad Frequency</asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</td>
<td class="style2" align="right">
<div id="div6" runat="server">
*</div>
</td>
<td align="left">
Ad Position:
</td>
<td align="left">
<asp:UpdatePanel ID="UpdatePanel6" runat="server">
<ContentTemplate>
<asp:DropDownList ID="ddlAdPos" runat="server">
<asp:ListItem Selected="True">Select Ad Positon</asp:ListItem>
</asp:DropDownList>
</ContentTemplate>
</asp:UpdatePanel>
</td>
</tr>
<tr>
<td align="left">
</td>
<td colspan="2" align="center">
 
</td>
<td align="left">
 
</td>
</tr>
<tr>
<td colspan="6" align="center">

<div id="divErr" runat="server">
</div>
</td>

</tr>
<tr>
<td align="left">
 
</td>
<td align="center">
<asp:Button ID="btnNew" runat="server" Text="New Rate Card" OnClientClick="return popNewRateWindow();" />
</td>
<td align="left">
<asp:Button ID="btnSave" runat="server" Text="Save Rate Card" OnClientClick="return validateDdlist();" OnClick="btnSave_Click" />

</td>
<td align="left">
 
</td>
</tr>
</table>
</div>
</form>
</body>
</html>

even i removed OnClick="btnSave_Click" still the problem is persisting ,please do the need full

thanks in advance