GridView In Webpart With Multiple Filters?

I'm currently working on a highly configurable Database Viewer webpart for WSS 3.0 which we are going to need for several customized sharepoint sites. but i fear it's necessary to recap the whole issue. As background information and to describe my problem as good as possible, I'll start by telling you what the webpart shall do:

Basically the webpart contains an UpdatePanel, which contains a GridView and an SqlDataSource. The select-query the Datasource uses can be set via webbrowseable properties or received from a consumer method from another webpart. Now i wanted to add a filtering feature to the webpart, so i want a dropdownlist in the headerrow for each column that should be filterable. As the select-query is completely dynamic and i don't know at design time which columns shall be filterable, i decided to add a webbrowseable property to contain an xml-formed string with filter information.

So i added the following into OnRowCreated of the gridview:

void gridView_RowCreated(object sender, GridViewRowEventArgs e)
{
if (e.Row.RowType == DataControlRowType.Header)
{
for (int i = 0; i < e.Row.Cells.Count; i++)
{
if (e.Row.Cells[i].GetType() == typeof(DataControlFieldHeaderCell))
{
string headerText = ((DataControlFieldHeaderCell)e.Row.Cells[i]).ContainingField.HeaderText;
// add sorting functionality
if (_allowSorting && !String.IsNullOrEmpty(headerText))
{
Label l = new Label();
l.Text = headerText;
l.ForeColor = Color.Blue;
l.Font.Bold = true;
l.ID = "Header" + i;
l.Attributes["title"] = "Sort by " + headerText;
l.Attributes["onmouseover"] = "this.style.cursor = 'pointer'; this.style.color = 'red'";
l.Attributes["onmouseout"] = "this.style.color = 'blue'";
l.Attributes["onclick"] = "__doPostBack('" + panel.UniqueID + "','SortBy$" + headerText + "');";
e.Row.Cells[i].Controls.Add(l);
}
// check if this column shall be filterable
if (!String.IsNullOrEmpty(filterXmlData))
{
XmlNode columnNode = GetColumnNode(headerText);
if (columnNode != null)
{
string dataValueField = columnNode.Attributes["DataValueField"] == null ? "" : columnNode.Attributes["DataValueField"].Value;
string filterQuery = columnNode.Attributes["FilterQuery"] == null ? "" : columnNode.Attributes["FilterQuery"].Value;
if (!String.IsNullOrEmpty(dataValueField) && !String.IsNullOrEmpty(filterQuery))
{
SqlDataSource ds = new SqlDataSource(_conStr, filterQuery);
DropDownList cbx = new DropDownList();
cbx.ID = "FilterCbx" + i;
cbx.Attributes["onchange"] = "__doPostBack('" + panel.UniqueID + "','SelectionChange$" + headerText + "$' + this.options[this.selectedIndex].value);";
cbx.Width = 150;
cbx.DataValueField = dataValueField;
cbx.DataSource = ds;
cbx.DataBound += new EventHandler(cbx_DataBound);
cbx.PreRender += new EventHandler(cbx_PreRender);
cbx.DataBind();
e.Row.Cells[i].Controls.Add(cbx);
}
}
}
}
}
}
}

GetColumnNode() checks in the filter property, if there is a node for the current column, which contains information about the Field the DropDownList should bind to, and the query for filling in the items.

In cbx_PreRender() i check ViewState and select an item in case of a postback. In cbx_DataBound() i just add tooltips to the list items as the dropdownlist has a fixed width.

Previously, I used AutoPostback and SelectedIndexChanged of the DDL to filter the grid, but to my disappointment it was not always fired. Now i check __EVENTTARGET and __EVENTARGUMENT in OnLoad and call a function when the postback event was due to a selection change in a DDL:

[Code]....

When i have two or more filters set which return zero rows, and i change back one filter to something that should return rows, the gridview remains empty (although the pager is rendered). I have to completly refresh the page to reset the filters. When debugging, i can see in the overridden CreateChildControls of the grid, that the base method indeed returns > 0, but anyway.the gridView.RowCount remains 0 after databinding. Anyone have an idea what's going wrong here?