Ajax Updatepanel with timer

Hi All
I have used AJAX Updatepanel with timer it was working previously but now its not working at all.
ie: the timer ticker event  is not calling after the particulare intervals.
Some time if i debug the code it goes the ticker event , if i remove the breakpiont , ticker event not called after particular intervals
this is my code
 
<asp:UpdatePanel runat="server" id="UpdatePanel1" updatemode="Conditional"> <Triggers> <asp:AsyncPostBackTrigger controlid="UpdateTimer" eventname="Tick" /> </Triggers> <ContentTemplate> <div id="dvBook" runat="server"> <img src="../images/general/spacer.gif" class="seat_book pad_0" alt="" /> <asp:HyperLink ID="hypbookseat" runat="server" NavigateUrl="Page2.aspx" Style="text-decoration: underline; padding: 0px;">C ...