UpdatePanel ScriptManager

Question

I Include a ScriptManager and two UpdatePanels; I inlcude a timer in UpdatePanel1 and interval is 100 ;and timer_tick is label1.text=datetime.now.tostring(); and I include a combobox in second UpdatPanel2; and combobox contains 3 items;

Now; I see the Date information in label1 it is ok; When I click ComboBox and I see the 3 items in downlist BUT in 1 second I lost the downlist immediatly; When I click again comboBox downlist shows 3 item and in 1 second the downlist hide again ;

What is the problem ? I don't understand.