<asp:UpdatePanel ID="up1" runat ="server" UpdateMode="Conditional" >
<asp:Label ID="label_time" runat="server" ></asp:Label>
<asp:AsyncPostBackTrigger ControlID="tim1" EventName="Tick"/>
The above code you are using will update only the content wrapped inside the update panel. The control that changes the question(next button) is located outside this update panel. So, that is causing the postback to update all the contents. You should wrap the contents to be updated asynchronously inside this update panel or you can use another update panel. That way the updation will become smooth and flicker free and only the contents inside the update panel will be updated.