Does it look anything like this?
http://i24.photobucket.com/albums/c16/n0otro/monitor.jpg
That is a picture of my G4 1.5ghz 15" powerbook.
Sometimes the lines across the screen are multicolored, sometimes they are black and white, sometimes its all white, it really varies. Generally it happens when I open up my powerbook. I have to play with the bezel and wiggle my monitor around to get it to "catch". I've also found that putting pressure directly on the left side of the screen next to the left side alleviates the problem. I usually try to wedge a piece of cardboard in between the bezel and the screen and that temporarily fixes the problem.
This appears to be a very common problem among powerbook owners over at the apple discussion forums, unfortunately apple won't recognize the problem and nobody seems to have a real fix.
If you go into system prefs --> displays --> select "Color LCD" and choose "open profile" --> scroll down until you find the 'mmod' tag
what information does it show?
Mine has:
Manufacturer: 00000610
Model: 00009C20
Serial: 00000000
Manufacture date: B87B8F78
It seems like alot of the people that are having these problems have the same model # so it seems to be a poorly manufactured piece of hardware.
Hope this sheds a little light