My PowerBook (model 5,6) has a defective display. Only the upper third shows the OpenFirmware or Ambient screen correctly, the rest of the screen is unreadable. For a while I'm using an external monitor which only works at home but is not handy when travelling. I turn the PowerBook on, close the lid, and when the external monitor shows Ambient, I open the lid again for better airflow.

Recently also on the external monitor the display started to show issues. Sometimes it immediately turns black, sometimes it comes back when I press some keys but often not. When the display show up again, it is usually trashed. (Up to here I would suspect the graphics chip.) Sometimes the keyboard also seems to be disconnected: when I hit CapsLock, the LED does not turn on. Sometimes the display comes back after I pressed Ctrl+lAmiga+rAmiga or even the power button instead of resetting the computer. (Here I would also suspect the logic board.)

Here are two pictures of how trashed the screen often looks like in case I can reactivate the display:
https://i.postimg.cc/6p6qSwHc/IMG-20220926-154225-small.jpg
https://i.postimg.cc/0Np4wRvg/IMG-20220926-154330-small.jpg

In either case I cannot read ther display clearly but after a reset it often works again. Sometimes I have to turn off the computer for a while and then it works again for a while.

The time for the blackout or trashes of the display to happen is very random, sometimes it takes not even a minute after system start, sometimes I can work for two hours.

Can someone make an educated guess what might be the reason for those effects? Overheated graphics chip? Issue on the logic board? Possibly a specific chip or are defective? How to figure out which one and is it worth doing so?
If such a guess gets close enough, te question is if it makes sense to repair this PowerBook and how, or whether I should buy another one instead.
The DVI cable and connector are fine, it must be anything internal to the PowerBook.