I would imagine Flash just isn't powerful enough to preform the operation you desire. I have never used such a large amount of rows on a DataGrid, so I'm no expert on this.
Here's a radical idea; add each row using a Timer, and trace each row as it is added to the DataGrid. Make the interval short, so it ticks quickly, but not fast enough to crash your program. I wonder where/if it will stop prematurely. It's worth a try since I imagine you do not want multiple DataGrids to hold you data.