Selection in datagrid gets really slow

Question

I am importing data from file into arraylists. Then I use the arraylists to fill a DataTable that I use as a DataSource for a DataGrid. But when I refill the Grid with new data from files, selection by pressing rowheader gets slower and slower. I get a delay before the entire row is highlighted. If I select a cell first, then the rowheader for the row containing that cell, the delay is not there. It only happens after refilling the table/grid a couple of times. It alså happens if I add columns to the table at run time (after adding about 5 columns it gets a noticable delay). What is up with that? -Atle