problema con ajax e autocomplete, va troppo in alto

[Kahm]

salve atutti
come potete notare in questa pagina: http://immobilzero.com/Prov2.aspx
ho una textbox ed una autocomplete
tutto funziona correttamente ma alla prima volta che appare l'autocomplete esso va troppo in alto!!
il problema non è l'animation, ho provato a cancellarlo e dava lo stesso errore, qui riporto il codice usato..da cosa puo dipendere?
grazie

<cc1:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
    <Services>
        <asp:ServiceReference Path="AutoComplete.asmx" />
    </Services>
</cc1:ToolkitScriptManager>
   
        <div>
        


            Type some characters in this textbox.  The web service returns random words
            that start with the text you have typed.
            


            
            <asp:TextBox runat="server" ID="myTextBox" Width="300" autocomplete="off" />
            [img]images/ajaxloader.gif[/img]
            
            <cc1:AutoCompleteExtender
             
                runat="server" 
                ID="autoComplete1" 
                TargetControlID="myTextBox"
                ServicePath="AutoComplete.asmx" 
                ServiceMethod="GetCompletionList"
                MinimumPrefixLength="2" 
                CompletionInterval="500"
                EnableCaching="true"
                CompletionSetCount="12" 
                onclientpopulating="ShowIcon"
                onclientpopulated="ShowIcon"
                BehaviorID="AutoCompleteEx" 
                >
                      <Animations>
                <OnShow>
                        <Sequence>
                                <OpacityAction Opacity="0" />
                                <HideAction Visible="true" />
                                <ScriptAction Script="
                                        // Cache the size and setup the initial size
                                        var behavior = $find('AutoCompleteEx');
                                        if (!behavior._height) {
                                                var target = behavior.get_completionList();
                                                behavior._height = target.offsetHeight - 2;
                                                target.style.height = '0px';
                                        }" />
                                <Parallel Duration=".4">
                                        <FadeIn />
                                        <Length PropertyKey="height" StartValue="0" EndValueScript="$find('AutoCompleteEx')._height" />
                                </Parallel>
                        </Sequence>
                </OnShow>
                <OnHide>
                        <Parallel Duration=".4">
                                <FadeOut />
                                <Length PropertyKey="height" StartValueScript="$find('AutoCompleteEx')._height" EndValue="0" />
                        </Parallel>
                </OnHide>
        </Animations>

                </cc1:AutoCompleteExtender>
                
               
    </div>
           
</form>