Boot the G4 from one of your myriad of OS X disks, preferably one that actually works, and see if the video problem remains.
If it does then you have ruled out a software problem.
If possible, try a different monitor. Does your graphics card have VGA as well as ADC connections? VGA is the smaller connection that you would normally find on a PC.
If the problem goes away, you've got your answer, but, if it doesn't, you still have your answer, the graphics card is failing.
Well done on the backup thing. Nice to hear that some people actually do things like that.
How is that OS 9 toting Mumsie?
Suddenly, when I boot up my computer, the screen on the monitor (albeit old) briefly shows the desktop, and then the image gets swallowed up into blackness. (That is literally what it looks like?not a sudden blackout, but the image collapsing inward as if into a black hole.) Occasionally, I can get things to work by various means, and I thought I had it licked, but this morning I just couldn't make anything work for more than an instant. Obviously, I can't do any work on that machine, even though the CPU shows the right light, the keyboard lights up when the computer is booted up, the mouse lights up when it's moved and so on.
I thought that this showed that I needed to replace my monitor, but one of my two sisters said that she thought it meant that my computer was failing. I am looking to you for advice.
The computer is a G4 (Sawtooth model) desktop with a separate monitor. It's running OS 10.4.11. The hard drive was replaced only a few years ago. I reinstalled and updated the operating system recently with a brand-new copy of Tiger. I haven't received any weird messages. Please ask for any other information you might need, but I hope that this will do for now. I'm on a laptop that is itself not in the greatest shape itself, and I hate to use it any more than I must. (I have kept it on OS 9 in order to be able to help my mother with any problems she might encounter on her computer, and I suspect that it wouldn't work well with OS X, anyway, with its smallish HD.)
Thanks for your input.
I backed up my files yesterday, so I'm not going to be in big trouble.