BusyIndicator and StealthPaging

Question

Hi ,
I am doing a SilverLight application using MVVM. In that I am calling webservice for populating datagrid using StealthPaging. While loading data , i am showing a 

busyindicator window which is a separate childwindow. It works perfectly for the first time. As i scroll down for retreiving the second set of data thru webservice, it

shows the busyindicator. The problem is after populating the data, ths scrollbar scrolls by itself. What can be the problem?
Please help. Thanks in advance.

xaml
----
<Grid Margin="-3,46,0,-622">
    <data:DataGrid  ItemsSource="{BindingPagedCollection,Mode=TwoWay}"                                 Name="dgridWithoutDetails"                            

VerticalAlignment="Top" Width="775" >
        :
        :
    </data:DataGrid>
</Grid>

ViewModel
---------
private void GetData(int StartRow, int EndRow)
{
    _IsBusy = new BusyIndicatorWindow();
    Singleton.Instance.ProgressMessage = Constants.PROGRESS_LOADINGDATA;
    _IsBusy.Show();

   
    // Calling WebService and populating collection

    _IsBusy.DialogResult = false;
}