Dell inspiron 8600. Screen goes black after a few seconds. External monitor is ok.

When I boot the screen goes black after a few seconds. Seems to bee during the dell image, before GRUB (ubuntu bootloader)

I have ubuntu 9.10 installed, and it acutally bootes fine. After hitting Fn+F8 the second screen displays ok. If I hit alt+printscrn+K , the ubuntu splash and logon screen is displayed ok on BOTH screens. But after a few seconds my laptop monitor goes black again. If I change resolution the laptop monitor is working for a few seconds. It seems to be stuck in some kind of power safe mode.

This started after I left the laptop on for a while and the screen had turned off when I got back.

I recently upgraded both memory and harddrive. Both have been working fine for a while.

I have a ati radeon 9600 graphic card.

Any suggestions would be higly appreciated!