WPF Listview flickering problem - wpf

Question

I am using a thread for reading the file and make a array of objects from the data read.A seperate thread for filtering records based on some filters is being used.
I am using listview to show data for dataset I get as per above.
While laoding a file everthing seems good but when I search,the problem exists.
while searching when I apply filters for the 1st time, everything is okay. Now again when I apply filters, data of the previous search is show for 1 sec(kind of flicker) and then the new data according to current filter is rendered.
I am using dispatcher for updating UI.
Before I bind any data, I try to clear ListView ItemSource by setting it to null
XAML code i am using is :
{
<ListView Name="listView1"
Grid.Row="0"
ItemsSource= "{Binding ElementName=Window1, Path=Entries}"
AllowDrop="True"
SelectionChanged="listView1_SelectionChanged"
Drop="listView1_Drop">
<ListView.View>
<GridView x:Name="GridView1" >
<GridViewColumn Header="Item" DisplayMemberBinding="{Binding Item}" />
<GridViewColumn Header="TimeStamp" DisplayMemberBinding="{Binding TimeStamp}"/>
<GridViewColumn Header="">
<GridViewColumn.CellTemplate>
<DataTemplate>
<Image Source="{Binding Image}" Width="16" Height="16" VerticalAlignment="Top" HorizontalAlignment="Left"/>
</DataTemplate>
</GridViewColumn.CellTemplate>
</GridViewColumn>
<GridViewColumn Header="Level" DisplayMemberBinding="{Binding Level}"/>
<GridViewColumn Header="Class" DisplayMemberBinding="{Binding Class}"/>
<GridViewColumn Header="Message" DisplayMemberBinding="{Binding Message1}" Width="250" />
</GridView>
</ListView.View>
</ListView>
}
Code Behind is :
private void ClearListView()
{
UpdateList update = new UpdateList(UpdateListData);
List<LogEntry> searcheddata = new List<LogEntry>();
searcheddata = null;
listView1.ItemsSource = null;
listView1.Items.Clear();
listView1.Items.Refresh();
pdDispatcher.BeginInvoke(update, DispatcherPriority.Render, searcheddata);
}
psdispatcher is the dispatcher used to update the UI & searchdata is function to assign datasource to listview