Sounds like hardware. Several possibilities come immediately to mind:
1) Backlight is going bad. (It's a fluorescent light).
2) Bad inverter
3) Loose or bad cable in laptop hinge
If the picture is OK using an external monitor, it's probably one of those three.
I've used these guys before for screen repair:
Unknown document title.
http://www.notebookmechanix.com/
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