Autopostback the page keeps refreshing

The DropDownList shouldn't be inside the ItemTemplate, as this means it will get "repeated" for each item. Because the DropDownList has AutoPostBack to true, and one static event handler, every time you select an item.

Move the DropDownList to outside the repeater, and it should solve your problem.
You'll need to wire up the SelectedIndexChanged event on the ItemCreated event.

This would then only re-render that segment of the page using ASP.NET AJAX.
http://msdn.microsoft.com/en-us/library/bb386454.aspx

I have a dropdownlist, and when I set AutoPostBack="true", the page keeps refreshing.
http://stackoverflow.com/questions/3749759/autopostback-keeps-refreshing-site