Dropdownlist with AutoPostBack true, page.ispostback still false in IE7 and IE8

Hi, I have a problem with Page.IsPostBack and AutoPostBack, Page.IsPostBack is always false after I make a change on dropdownlist which has AutoPostBack = true, this only occurs in IE7 and IE8, other version of IE is fine, this is not in a special control and not in a gridview. Don't have any error message or javascript error. Please help, thanks!

code in aspx:

<asp:DropDownList ID="ddlConnectedPartyType" runat="server" Height="20px" AutoPostBack="true" AppendDataBoundItems="true"                                OnSelectedIndexChanged ="ddlConnectedPartyType_SelectedIndexChanged" ViewStateMode="Enabled"
EnableViewState="true"                                Width="308px" EnableTheming="false"
DataSourceID="GetConnectedPartyType" DataTextField="RPTypeDesc"                                DataValueField="RPTypeCode">                                <asp:ListItem Text="" Value="" Selected="True"/>                        </asp:DropDownList><asp:ObjectDataSource ID="GetConnectedPartyType" runat="server" TypeName="PatriotOfficer.NameCheck_DataSource_TW"        SelectMethod="GetConnectedPartyType">    </asp:ObjectDataSource>

Page_Load in code behind:

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load            If Not Page.IsPostBack Then                MultipleList_Data.SelectParameters.Clear()                MultipleList_Data.SelectParameters.Add("Action", "TRUNCATE")                MultipleList_Data.SelectParameters.Add("Extra_Name", "")                MultipleList_Data.SelectParameters.Add("TAXID", "")                MultipleList_Data.SelectParameters.Add("Nationality", "")                MultipleList_Data.SelectParameters.Add("DOB", "")                MultipleList_Data.SelectParameters.Add("AddrCountry", "")                MultipleList_Data.SelectParameters.Add("AddrCity", "")                MultipleList_Data.SelectParameters.Add("Address", "")                MultipleList_Data.SelectParameters.Add("RPTypeCode", "")                MultipleList_Data.SelectParameters.Add("RPSubTypeCode", "")            End If        End Sub

SelectedIndexChanged in code behind:

Protected Sub ddlConnectedPartyType_SelectedIndexChanged(ByVal
sender As Object, ByVal e As System.EventArgs) Handles
ddlConnectedPartyType.SelectedIndexChanged            txtConnectedPartyType.Text = ddlConnectedPartyType.SelectedValue            ddlConnectedPartySubType.Items.Clear()            ddlConnectedPartySubType.Items.Add(New ListItem("", ""))            ddlConnectedPartySubType.DataBind()        End Sub