Fast/Bulk Postback handling

I have a search feature that is similar to the google search autocomplete.  I have a textbox that the user enters their search criteria, then with javascript I am automatically clicking a hidden button inside an updatepanel so I can run code on the server.  This all works great until somebody leans on the keyboard and it can't handle that many postbacks is my guess (but it could be something else).

The error I am randomly getting is "The state information is invalid for this page and might be corrupted.".

I made it error less when I try to determine if it is in the middle of a postback by using a variable, but it still errors sometimes:

 If iDuringPostback.Value.ToString = "Yes" Then
        Else
'Refresh Results
End If

Here is the code I am using to add the onKeyUp event:

If Not IsPostBack Then
                txtSearch.Attributes.Add("onKeyUp", "doKeyUp('" + btnSearchHidden.ClientID + "','" + iHiddenSearchValue.ClientID + "',event)")

Adding enableeventvalidation= false in the web.config does not fix the problem.   Any ideas?

EDIT: It appears to only be a problem in IE