I tried to install an Acer flat screen monior to my Dell laptop. Flat screen work for 10s then black...what?

I have a Dell Inspiron E1405 with XP. When I hit FN key plus F8 Flat screen shows for 10sec then goes black. My laptop screen is fine just trying to add the flat screen. Any ideas why the flatscreen only shows my laptop screen mirror for 10sec? How can I make it work normally and have the flatscreen on for all time instead of 10sec?