Vostro 1700 - black screen on startup

When I received my Vostro 1700, I had to connect it to the mains before I could see the Dell boot screen. The battery was fully charged. Sometimes, I was able to boot normaly on battery power and when the screen remained black, I had to connect to the mains in order to get a working screen. A week later I got a black screen even when connected to the mains. I know the backlight is ok because when using the Fn+Up or Fn+DOWN keys, I kan see the screen going darker and brighter from the light leakage. So now I havne to use Ctrl-Alt-Delete during the boot sequence until I get the Dell boot screen. I've run the diagnostics but nothing was found. Is there any sollution to this problem?