Databound strange behaviour with large amount of data

I'm experiencing a strange behaviour of a <AjaxToolkit:Combobox> widget used on Asp.net form.

I got it declared this way:

<ajaxToolkit:ComboBox
    ID="cbConvenzionato"
    runat="server"
    AutoCompleteMode="SuggestAppend"
    DropDownStyle="DropDownList"
    DataSourceID="sdsConvenzionati"
    DataTextField="nome"
    DataValueField="id"
    AutoPostBack="true" 
    Width="200px" 
    OnSelectedIndexChanged="cbConvenzionato_SelectedIndexChanged" 
    AppendDataBoundItems="true"><asp:ListItem Text="Tutti" Value="" Selected="True" /></ajaxToolkit:ComboBox>

And its related data-source in this way:

<asp:SqlDataSource
    ID="sdsConvenzionati"
    runat="server"
    ConnectionString="<%$ ConnectionStrings:db %>"
    ProviderName="<%$ ConnectionStrings:db.ProviderName %>"
    SelectCommand="
        SELECT
            id,
            nome
        FROM
            anag_convenzionati
        WHERE
            ((data_cancellazione IS NULL) OR (data_cancellazione = ''))
        ORDER BY nome;" />

I think that the problem is given from the amount of data provided from the datasource since I used the same approach many times and I never had such kind of issues.

The amount of raw data transfered for such table should be around 100k (calculations made exporting data from database and encoding it the way JSON should).

The abnormal behaviour is not on the data visualization itself since combobox appears to be filled with all the data but from the selection mechanism that brokes and does not even allow to enter characters inside the widget. The only way toselect items is trough graphical interaction (that may takes time since there are lots of items).

Threre's a way to fix this?

P.S. As a side question I would like to know if there's a way to change the way the item selection mechanism works i.e. I need a control that allows to search for a substring instead left string matching.

Best regards,

 Mike