hi, i had the same problem with a dell latitude d610. Problem was a part called an 'inverter' it had something to do with starting the backlight and mine had the very same symptoms as your's. I purchased the part from ebay for about 15euro's and replaced it myself no problems, this involved taking half the laptop apart though my laptop had no warranty so i didnt mind. Also ive heard of the switch that turns off the monitor backlight when you close the lid can sometimes get stuck. If its still under warranty then probly best to get it repaired by the vendor though. Hope this helps.