Hi,

This behavior could only be observed in SL and it is caused by the refreshing Visibility of the PaneGroups. When the Group is initialized it is initially Visible and after it gets completely loaded its Visibility and status of its Panes (remove them if their IsHidden property is set to True) get updated - if there are no visible items in the Group the PaneGroup is Collaped which causes the observed flickering of the layout.

In order to workaround this unexpected behavior you could create a class that derives from the RadPaneGroup class and override its OnApplyTemplate method. Inside it to  create same custom logic for Collapsing initially the PaneGroup. Thus the PaneGroup will be Collaped initially and will be visualized after it gets loaded  - the initial position that is immediately hidden should not be visualized:

public class CustomPaneGroup : RadPaneGroup
{
    public override void OnApplyTemplate()
    {
        base.OnApplyTemplate();

        if (this.Items.Count == 1 && (this.Items[0] as RadPane).IsHidden)
        {
            this.Visibility = System.Windows.Visibility.Collapsed;
        }
    }
}

Regards,
Nasko