Ajax is not working after few random drop down select and textbox text type

Dear Expert,

I have created very simple application to select some data from dropdown and next to type and add in dropdown item. Without ajax exerything fine and never receive any problem with these code. But problem happening if I set these control in UpdatePanel (ScriptManager in page but control outside UpdatePanel then no problem). When I am selection these dropdown randomly (these are auto post back enable and casecase dropdown type) after 3-4 selection of dropdown nothing is happening. I have tested in debug mode these are firing postback event. However same thign never creating problem for nonajax code.

Any idea why happening? I have taken correct AjaxToolkit DLL.

Thanks in advance

Suman