Ajax Modal Popup Extender problem

I am using AjaxModalPopupExtender to show popup in my web application.

my coding sample is follows:

<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
        </asp:ToolkitScriptManager>

<asp:UpdatePanel ID="ButtonUpdatePanel" runat="server" UpdateMode="Conditional">
                                <ContentTemplate>
                                    <asp:Button ID="Button1" OnClick="Button1_Click" CssClass="transfer" runat="server" Text="Button1" />
                                </ContentTemplate>
                            </asp:UpdatePanel>

<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender1" runat="server"
TargetControlID="btnModalPopup"
PopupControlID="Panel1"
BackgroundCssClass="modalBackground"
CancelControlID="btnCancel"
PopupDragHandleControlID="dragHandle"
DropShadow="true" Enabled="true">
</ajaxToolkit:ModalPopupExtender>

<asp:LinkButton ID="btnModalPopup" runat="server" style="display:none;"></asp:LinkButton>

<asp:Panel ID="Panel1" runat="server" Style="display:none">

   <div ID="dragHandle" runat="server">Popup Title</div>

Panel Content...................

   <asp:Button ID="btnOK" OnClick="btnOK_Click"...>

   <asp:BUtton ID="btnCancel" OnClick="btnCancel_Click".....>

</asp:Panel>

Codebehind

protected void Button1_Click(object sender, eventargs e)

{

     //codes

    ModalPopupExtender1.show();

}

protected void btnOK_Click(object sender, eventargs e)

{

     //saving to database and other codes

    ModalPopupExtender1.hide();

}

protected void btnCancel_Click(object sender, eventargs e)

{

     //codes

    ModalPopupExtender1.hide();

}

now I have one problem.

Panel1 (modalPopupPanel) appears for short while on page refresh or on the click event of another button.

it always happens on the first time.

after the first time, it is running smooth.

As you can see my coding, I carefully set "display:none" for this panel.

it should appear on the click event of target button,

otherwise it must be hidden nomatter page reload or other button click.

how should I solve?

pls check my coding wheather it is still wrong somewhere?

running on asp.net 3.5

FireFox

regards