Listview Grouping - IsGroupingEnabled

My question is simple:

Which steps do i have to do to activate and deactivate the grouping from a listview at runtime? IsGroupingEnabled = false

I have a perfect grouped Listview with its Itemsource bound to an observable collection of a groupviewmodel.

But the moment i decide to deactivate the property 'IsGroupingEnabled', i see a blank list without any functional template.
Every binding seems to be gone and the list presents itself with a list and each item.tostring();

I already found out out that i definatley have to remove the Listview.GroupHeaderTemplate = null
otherwise it still exists.

Interesting fact, if i set IsGroupingEnabled to 'false' before the listview is generated the firsttime the same datatemplate works for me...

Or do i have to use a second listview, beause it will never get rid of the template cache ?