Problem with Ajax Toolkit Popup

Question

Hello all,
Can you help me about ajax toolkit popup?
when I run program and click button for show Popup it always show messagebox as below.




in this popup have one TreeView it get data from table that have 1000 record. if this popup not get data from database it can show smoothly.
and other way, when run it in Mozilla it can show smoothly too. I try to fix it but can not.
you can help me to solve it? or you can write code that good for it in get alot of data.

Please see some code below:
<ajaxToolkit:ModalPopupExtender ID="PanelLocation_ModalPopupExtender"
                        runat="server"
                        DynamicServicePath=""
                        Enabled="True"
                        TargetControlID="btnSearch"
                        BackgroundCssClass="modalBackground"
                        PopupControlID="PanelLocation"           
                        CancelControlID="btnCalcelLocation"
                        DropShadow="false">
                  </ajaxToolkit:ModalPopupExtender>
                   
              <%--   <div style=" height:0px">--%>
                    <asp:Panel ID="PanelLocation" Style="display: none;" runat="server"  CssClass="dplpanel" Width="370" Height="285" Enabled="true">
                      <div id="con_popup"><table width="100%" cellpadding="0" cellspacing="0"><tr ><td class="cur_ti" width="3"><img src="imgs/cur_sym.png" /></td><td class="cur_ti">Location Code</td><td width="25" align="center" valign="middle" >
                      <asp:ImageButton ID="imbClose" ImageUrl="imgs/close.gif" runat="server" /></td></tr></table></div>
                     
                        <div style=" margin:5px; padding:1px; padding-left:5px; border:1px solid #a3b3c7;">
                            <asp:Panel ID="Panel4" runat="server" ScrollBars="Vertical" Height="200">          
                                <asp:TreeView ID="TreeView1" runat="server"
                                    onselectednodechanged="TreeView1_SelectedNodeChanged" 
                                    Width="259px" ImageSet="Arrows">                       
                                    <ParentNodeStyle Font-Bold="False" />
                                    <HoverNodeStyle Font-Underline="True" ForeColor="#5555DD" />
                                    <SelectedNodeStyle Font-Underline="True" ForeColor="#5555DD"
                                    HorizontalPadding="0px" VerticalPadding="0px" />
                                    <NodeStyle Font-Names="Tahoma" Font-Size="10pt" ForeColor="Black"
                                    HorizontalPadding="5px" NodeSpacing="0px" VerticalPadding="0px"  />
                                </asp:TreeView>
                            </asp:Panel>            
                                   
                        </div>
                   <div style=" padding-left:10px; height:15px;"><asp:Label ID="lblLocationValue" runat="server" Text=""></asp:Label>
                         <asp:Label ID="lblLocationText" runat="server" Text=""></asp:Label></div>  
                   <div style=" float:right" class="pn">
                        <asp:Button ID="btnOKLocation" runat="server" Text="OK" CssClass="btn" onclick="btnOKLocation_Click" />
                        <asp:Button ID="btnCalcelLocation" runat="server" Text="Cancel" CssClass="btn" />
                   </div>
            
               </asp:Panel>   



Thank Advance.