Many thanks for any thoughts you may have on this!

My G4 Powerbook started suddenly having the screen go black randomly. I could restart and it would be ok for a little while. I was assuming maybe it was something wrong with the screen, but now I see that's not the case at all. The latest is that when I have it hooked up to an external monitor, it will boot to the point of the apple screen and then suddenly shut off. It does the exact same behavior in Target Disc Mode---I get the firewire symbol for a few seconds, and then it shuts off.

Except when it doesn't. And by that I mean, it also will often not start up at all, though it doesn't shut itself all the way off either. I often have to take out the battery and the A/C power before the power really dies. Then when I push the button to start it up again, it will sometimes just make a kind of defeated little sound (definitely not a chime) and not come on. Though, again, I usually have to undo battery and power to get it to shut down.

The fan sometimes comes on, sometimes not.

Interestingly, when it did come on in Target Disk Mode briefly before it shut itself down again, I did see all the volumes from the laptop showing up on other computer's desktop as they should, with correct icons and everything. This makes me hopeful that it's not the hard drive. Then again, maybe it's all the rest of the laptop innards!

I've got a call out to borrow a power cord to make sure it's not part of the problem. Also, I've tried resetting the PMU a number of times.

I'm really perplexed. It's just so very, very random, and increasingly, it's very hard to get it to start up at all, which makes troubleshooting difficult (trying to start up from the CD was a total fail).

Definitely a problem for a guru and not a mere mortal like me.
Many thanks!