Populate and clear list in a task

I'm thinking about the DataGrid. When I have the issue, the following error message is showed :

An unhandled exception of type 'System.IndexOutOfRangeException' occurred in mscorlib.dllAdditional information: Index was outside the bounds of the array.

I use a customized observable collection as source for the DataGrid. Each time that an item is added to/remove from the collection, a line is added to/removed from the DataGrid.

I cound have a problem of sync between the list and the control...