Many PowerBooks having serial numbers starting with "W8" suffer a history of bad displays. There is in this forum a massive thread on the subject. To verify it is the display at fault and not the video hardware on the logic board, connect and external monitor to the DVI port and see if the problem repeats. If not, it's a bad display. If it repeats, it's more serious, requiring a logic board changeout.
Even the cheaper option of only the display being bad may cost more to repair than a used PowerBook is worth. Tread cautiously.
Mine had a vertical line running through the center of the display. Didn't look too bad until another line appear some time after.
I replaced the display myself, but didn't bother with taking the actual display panel from the housing.
It's lot more easier to replace the screen with the housing intact. I got one that was working from eBay for £50 ($76) It was a simple case of removing the top cover (keyboard), unpuagging the monitor cables (Airport cables) and unscrewing the display. Lift away the old one and replace with the replacement one.
Display removal instructions can be found here:
With the right tools and careful attention, your PowerBook G4 display can be easily fixed for less than $100 (£60)
Since then I've had no further problem with my PowerBook.
Hope this helps.