PowerBook - Powerbook G4 Screen issue

Im posting this on behalf of a friend of mine who is having an issue with his powerbook G4 running leopard 10.5.

He is getting a really jumbled display and most of the time you cant make out anything on the screen at all. What really gets me is that sometimes you can turn on the computer and the display will be crystal clear and then after a little while it will get all screwed up again.

I was thinking that it was just a bad display so i brought over a monitor for him to use for the time being. Upon connecting the monitor to the laptop the screen looked exactly the same as it did on the actual laptop display. This leads me to believe that he may have a hardware issue of some kind. Maybe there is some kind of corruption in the actual video card that is screwing up the display. I was thinking it was some sort of corruption with he card or perhaps that there is a cooling issue. Maybe the fan on the card is going dead or some kind of heat-sync which causes the jumbled display when the system heats up.

Any ideas on this would be great, thanks...