PowerBook - Ti DVI horizontal lines on the screen

Ok, this only happens when i view videos on the web, mainly the flash based ones from youtube etc. so after a few minutes of viewing videos, i get shadows, and horizontal lines.
none of my pixels on my screen are dead.

if i close the window, or minimize it, the screen will go back to normal after 20-30 seconds or if i hit the detect monitors button.

i have hooked up an external monitor to the Powerbook and the other screen is fine.

I can view and open quicktime, wmv, divx, realplayer files fine- but when it comes to viewing flash based videos or sometimes DVDs the horizontal lines will occur.

what is going bad and what do i need to replace?

Ti DVI 1GHz
Running 10.4.9 (also happened with 10.2.8 installed)