modalpopup gets opened or displays behind the page.

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ASP.NET

Javascript

Ajax

Dear members,

I am using modalpopup extender on one of my page.
i have mentioned modalpopup css and modalpopupbackground,behaviourid

The problem is modalpopup gets displayed properly for first 5 time, as soon as i click it for 6 or more, modalpopup gets displayed behind the main screen,or goes behind the page.
this problem occurs when i continuously open and close modal popup multiple times without performing some other action.

for opening modalpopup i have set linkbuttonname to targetcontrolid and for closing a popup i have
used a cancel button serverside event to close it . Modalpopup1.close()

After doing google, i did mentioned some z-index property to 1 and even tried with 10000+ values, but still the problem persist,

Below is the code

.modalPopup
{
    background-color: White;
    border: 0;
    vertical-align: top;
    z-index:10001 !important;
}
.modalBackground
{
  background-color: Gray;
    filter: alpha(opacity=70);
    opacity: 0.7px;


}

<asp:UpdatePanel ID="Upnl_SelLineItems" runat="server">
                                                                                            <ContentTemplate>
                                                                                                <asp:LinkButton ID="lnkSelLineItem" runat="server" Text="Basket"></asp:LinkButton>
                                                                                                <asp:Panel ID="pnl_SelLineItem" runat="server" CssClass="modalPopup">
                                                                                                    <table style="border: thin solid #006699;">
                                                                                                        <tr>
                                                                                                            <td style="text-align: center; background-color: #006699; color: #FFFFFF;">
                                                                                                                Selected Line Items
                                                                                                            </td>
                                                                                                        </tr>
                                                                                                        <tr>
                                                                                                            <td align="center">
                                                                                                                <asp:Panel ID="pnl_grdSelLineItem" runat="server" ScrollBars="Auto">
                                                                                                                    <asp:GridView ID="grd_SelLineItem" runat="server" AutoGenerateColumns="false" CssClass="Grid"
                                                                                                                        EmptyDataText="No Line Items Selected">
                                                                                                                        <Columns>
                                                                                                                            <asp:BoundField DataField="JEDate" HeaderText="JE Date" ReadOnly="True" />
                                                                                                                            <asp:BoundField DataField="JENumber" HeaderText="JE No." ReadOnly="True" />
                                                                                                                            <asp:BoundField DataField="DocumentType" HeaderText="Document Type" ReadOnly="True" />
                                                                                                                            <asp:BoundField DataField="FunctionalCurrency" HeaderText="Currency" ReadOnly="True" />
                                                                                                                            <asp:BoundField DataField="FunctionalAmount" HeaderText="Amount" ReadOnly="True" />
                                                                                                                            <asp:BoundField DataField="Description" HeaderText="Description" ReadOnly="True" />
                                                                                                                            <asp:TemplateField Visible="false">
                                                                                                                                <ItemTemplate>
                                                                                                                                    <asp:Label ID="lblauto_id" runat="server" Text='<%#Bind("Auto_Id") %>' Visible="false"></asp:Label>
                                                                                                                                </ItemTemplate>
                                                                                                                            </asp:TemplateField>
                                                                                                                            <asp:BoundField DataField="PeriodStatus" HeaderText="PeriodStatus" />
                                                                                                                            <asp:BoundField DataField="PeriodComment" HeaderText="PeriodComment" />
                                                                                                                        </Columns>
                                                                                                                    </asp:GridView>
                                                                                                                </asp:Panel>
                                                                                                            </td>
                                                                                                        </tr>
                                                                                                        <tr>
                                                                                                            <td valign="top" style="text-align: center">
                                                                                                                <asp:Button ID="btnClose" runat="server" Text="Close" />
                                                                                                            </td>
                                                                                                        </tr>
                                                                                                    </table>
                                                                                                </asp:Panel>
                                                                                                <asp:ModalPopupExtender ID="mpex_SelLineItem" runat="server" TargetControlID="lnkSelLineItem"
                                                                                                    BackgroundCssClass="modalBackground" PopupControlID="pnl_SelLineItem" RepositionMode="RepositionOnWindowResizeAndScroll"
                                                                                                    BehaviorID="B3" DropShadow="true" PopupDragHandleControlID="pnl_SelLineItem">
                                                                                                </asp:ModalPopupExtender>
                                                                                                <asp:HiddenField ID="HiddenField3" runat="server" />
                                                                                            </ContentTemplate>
                                                                                            <Triggers>
                                                                                               <asp:AsyncPostBackTrigger ControlID="btnClose" EventName="Click" />
                                                                                            </Triggers>
                                                                                        </asp:UpdatePanel>

Any suggestions?