Have you used the
asp:UpdatePanel
?
When I am working with Drop Down Lists and Textboxes and ASP.NET I use Update Panels around the control that I want to update and then set the trigger to be the control (in your case the Textbox perhaps?)
Try surrounding this area of yous page with the update panel, and see how that works.
I wonder if this article posted on CodeProject will help you.
http://www.codeproject.com/KB/webforms/GridViewFilter.aspx[
]
Good luck
Rob