Datagrid - MultiValueConverter called twice on Refresh

Hello,
I'm using a Datagrid bound on a collection of custom objects, with 20 columns.
I also use a MultiValueConverter  to change the background color of each cell based on some different rules.
When the user clicks on some buttons, these rules change and the Datagrid cells background color are changed. My collection never changes.
The problem is that when I call MyDatagrid.Items.Refresh(), the background converter is called twice for all visible cells.
It's the same behavior when I call CollectionViewSource.GetDefaultView(MyDatagrid.ItemsSource).Refresh();
 When I call MyDatagrid.InvalidateVisual(), or UpdateLayout(), the visible cells are not changed until I use the scroll bar.
Several grids need to be changed, and it takes about 190 ms for each grid to be refreshed. That's too much. 
I need to invalidate visible cells only once for performance reasons.
How can I do that ?
My project : WPF, target : framework 4.0
Thanks in advance