asp.net ModalPopupExtender show's then hides problem

I have a simple ModalPopupExtender sample but can't seem to get it working properly. I've followed the code from an online sample and it's similar to a lot of the other online examples but when I run it, the popup panel will quick show then then hides again. I've set the targetcontrol to a linkbutton and when I click it it will show the popup, triggers a postback then hides it afterwards thus causing a quick flicker.

here is my code...

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

<asp:LinkButton ID="LinkButton1" runat="server">Add Item</asp:LinkButton>

<asp:P开发者_开发问答anel ID="PanelPopup" runat="server" CssClass="ModalPopup" Width="500" Height="350" style="display:none;">
    <h1>test</h1>
    <asp:Button ID="ButtonAdd" runat="server" Text="Add" />
    <asp:Button ID="ButtonCancel" runat="server" Text="Cancel" />
</asp:Panel>

<asp:ModalPopupExtender 
    ID="ModalPopupExtender1" 
    runat="server" 
    BackgroundCssClass="Inactive"
    PopupControlID="PanelPopup"
    TargetControlID="LinkButton1"
    OkControlID="ButtonAdd"
    CancelControlID="ButtonCancel">
</asp:ModalPopupExtender>

I've also added Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" at the top

Can anyone see anything different or wrong about this please? Many thanks!

It might be because your using the ASP ScriptManager. Try using the ToolkitScriptManager that comes with the AJAXControlToolkit.