So, this only started about a month ago, and it has gradually gotten way worse.

I have a Titanium Power Book G4.

Basically, any time I watch a video for too long, be it on the internet, DVD, or anything, after a couple minutes the entire screen starts twitching, and multi-colored lines randomly appear all over the screen. They quiver and change, but then they go away the second I turn the video off.

Any help?