Gridview properties SortExpression and SortDirection not maintaining values when using paging...

1) I am manually binding to a MySQL database
2) Therefore I am manually handling the firing of the Paging and the Sorting events in codebehind.
3) I do a sort on Column.  It sorts fine...
4) I click on say Page 4.. The sorting is completely lost but goes to page 4 of the default sorting (as if the page was loaded the first time and no sort was indicated)

I don't understand why the gridview maintains the PageIndex Value and not the SortExpression and SortDirection values when posting back...For example, if I click on Page 4 and then click a new column to sort.  The GridView.pageIndex holds it value.

But if I click on a column header for sorting and then click on page 4.  The gridview.sortdirection and gridview.sortexpression are reset....

ASPX
[code]
<asp:ScriptManagerProxy ID="ScriptManagerProxy1" runat="server" />
        <asp:UpdatePanel ID="upEmployeeList" runat="server" UpdateMode="conditional" EnableViewState="true">
            <ContentTemplate>
            <asp:GridView cellpadding=5 ID="gvEmployeeList" runat="server"
                AutoGenerateColumns="false" PageSize="20" AllowSorting="true" AllowPaging="true"
                OnPageIndexChanging="gvEmployeeList_PageIndexChanging"
                OnSorting="gvEmployeeList_Sorting"
                OnRowCommand="gvEmployeeList_RowCommand"
                EnableViewState="true"
            >
                <Columns>
                    <asp:BoundField DataField="EmpId" HeaderText="Emp ID" SortExpression="EmpId" ReadOnly="true" />
                    <asp:BoundField DataField="Last_Name" HeaderText="Last Name" SortExpression="Last_Name" ReadOnly="true" />
                    <asp:BoundField DataField="First_Name" HeaderText="First Name" SortExpression="First_Name" ReadOnly="true" />
                    <asp:BoundField DataField="Manager" HeaderText="Manager" SortExpression="Manager" ReadOnly="true" />
                    <asp:BoundField DataField="Division" HeaderText="Division" SortExpression="Division" ReadOnly="true" />
                    <asp:BoundField DataField="Department" HeaderText="Department" SortExpression="Department" ReadOnly="true" />
                    <asp:BoundField DataField="Floor" HeaderText="Floor" SortExpression="Floor" ReadOnly="true" />
                    <asp:ButtonField ImageUrl="~/admin/images/grid_edit.gif" CommandName="edit" ButtonType="Image" />
                    <asp:ButtonField ImageUrl="~/admin/images/grid_delete.gif" CommandName="delete" ButtonType="Image" />
                </Columns>
                <HeaderStyle CssClass="gridHeader"  />
                <RowStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Left" VerticalAlign="Middle" />
                <SelectedRowStyle BackColor="#738A9C" Font-Bold="True" ForeColor="#F7F7F7" />
                <PagerStyle BackColor="#E7E7FF" ForeColor="#4A3C8C" HorizontalAlign="Right" />
                <AlternatingRowStyle BackColor="#F7F7F7" HorizontalAlign="Left" VerticalAlign="Middle" />
            </asp:GridView>            
        </ContentTemplate>
[/code]
c# file
[code]
    private string sSortDirection="ASC";
    private string sSortExpression="EmpId";

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            gvEmployeeList.DataSource = ds_gvEmployeeList();
            gvEmployeeList.DataBind();
        }
    }
    protected DataSet ds_gvEmployeeList()
    {
        string sSQL = "SELECT a.*,  CONCAT(b.Last_Name,', ',b.First_Name) as Manager, d.NAME as Division, c.NAME as Department ";
        sSQL = sSQL + " from employee a ";
        sSQL = sSQL + " LEFT JOIN Employee b ON a.MGRID=b.EmpID ";
        sSQL = sSQL + " LEFT JOIN Department c ON a.DEPTID=c.DEPTID ";
        sSQL = sSQL + " LEFT JOIN Division d ON a.DVSNID=d.DVSNID ";
        sSQL = sSQL + " ORDER BY " + sSortExpression + " " + sSortDirection;

        DataSet dsTemp = new DataSet();
        using (OleDbDataAdapter da_gvEmployeeList = new OleDbDataAdapter(sSQL, sConnect)){
            da_gvEmployeeList.Fill(dsTemp, "EmployeeList");
        }
        return dsTemp;
    }
    protected void gvEmployeeList_PageIndexChanging(Object sender, GridViewPageEventArgs e)
    {
        gvEmployeeList.PageIndex = e.NewPageIndex;
        gvEmployeeList.DataSource = ds_gvEmployeeList();
        gvEmployeeList.DataBind();
    }
    private string GetSortDirection(string sSortDirection){
        switch (sSortDirection)
        {
            case "ASC":
                return "DESC";
            case "DESC":
                return "ASC";
            default:
                return "ASC";
        }
    }
    protected void gvEmployeeList_Sorting(Object sender, GridViewSortEventArgs e)
    {
        sSortDirection = GetSortDirection(e.SortDirection.ToString());
        sSortExpression = e.SortExpression;
        gvEmployeeList.DataSource = ds_gvEmployeeList();
        gvEmployeeList.DataBind();
    }

any help is greatly appreciated!

Yours in deepest gratitude,

Angela Law