iMac G5 lock up - big problem

Just a day or two ago, I noticed my computer had started to freeze up after it's been on the screensaver for a while. Sometimes the mouse cursor would still move (the screensaver image would be froze) and sometimes there would be no curser at all. Nothing was running in the background other than things like mail, iChat, ect. So it was completely normal use which has never been a problem before.

I have to hold in the power button to get it to turn off to restart the computer. No keyboard commands would do anything. Sometimes, it would boot up completely, and other times it would freeze booting up, before the MacOSX screen loaded.

Once, it was froze up when it got to the finder. When it boots fully and the finder isn't froze, the computer works normally. Although now I'm finding that after a while, the computer locks up while I'm using it, and not just when it's idle. I have tried using a different user acount and the same thing happens.

Also, the fans come on full blast sometimes after it locks up, and when it's been rebooted. I ran the apple hardware test, and it said everything passed and was fine. I can not think of anyting I have downloaded or installed in the past few days. I am trying to copy some recording sessions off the disk that I have done for work and need to be sure to not loose, but so far it's locked up before the transfer to a firewire drive has completed. I've tried booting to target disk mode and copying to another computer, but the transfer rate was for some reason incredibly slow and basically not moving.

Hopefully I can get these files copied, and then I will try to reinstall OSX system software and HOPEFULLY that will take care of the problem. If not, I'll try and back everything up, reformat and reinstall.

It is such a strange situation, it doesn't really seem to me to make sense that it's a software problem, but at the same time I don't know what hardware problem it would be either. The confusing thing to me is the fans coming on full blast, after it's froze - which is what makes me think it's posibly a hardware problem.

Anyone else have similar problems or heard of this? It's an iMac G5 2ghz.
Any ideas would be helpful thanks!

i've just got the stock 512 ram that came with the computer. I took the back off in hopes of pulling the hard drive out to put it in a FW case to back it up, but I don't have the right screw driver to get it out. It wasn't very dusty but I blew what was there out.

I really don't see how it could be an overheating issue that is locking it up, because the fans aren't even on until after it's been sitting there froze for a while anyways.

But who knows.
At least I still have a bit of the warrentee left if I can't get it figured out.

hrm... I did have a USB iSub and a couple firewire HD's plugged in, although sometimes they were on sometimes they were off. Plus, they had been plugged in in the past. But right now I'm not using the bluetooth keyboard, just a usb mouse and keyboard, and one firewire drive and so far it hasn't locked up while coping the file over. It actually just finished copying, took 45 min maybe. So maybe it was all the stuff plugged in.

Tomorrow I'll reinstall OSX and see if that seems to solve it. But this still isn't something that should be happening. I'm not really happy about it, though it isn't that big of a deal right now, but it easily could be in the future if this happens while I'm working recording live concerts in which case, would be a HUGE problem. Plus, I'd be pretty upset if it happens again (assuming reinstall will fix it) after the warrantee is up and I'm left with a big hardware issue.

Running 10.4.6. Although, it managed to stay on all night... so maybe it worked itself out.

so far today, it hasn't had any problems. I got the files I really needed copied off, but I haven't reinstaled OSX or anything, but it seems to be fine so far. Screen saver and all... who knows. 24 hour flu?

UPDATE:
Nope, now it's locked up while I was gone. Looks like I'll be reinstalling OSX.

UPDATE 2:
I locked up a few times while installing OSX again. I booted into single user mode and my brother poked around and it showed that the hardware wasn't running very hard, and then the fans came one. So something is up.

I appears to lock up, only when I have the computer on the network, be it the airport or eithernet. That is, once the comptuer is booted to OSX, it only locks up when it's on the network. It does sometimes lock up trying to reboot if it hasn't been sitting for 10min or so before booting up after a freeze.

thanks for all the suggestions and ideas. I'd reformated the comuter and installed from the dics it shipped with, and still locked up without using the network. So i took it into the local repair place that's apple certified or whatever. He said he didn't think it sounded like a software problem, and said he was going to loop the hardware test for like 12 hours o rsomething.

So I dunno, hopefully they can figure it out. And if they find/claim it's a software issue I"m gonna be really upset because then they will charge me even though it's under warrentee, which I"m sure if I drove 2.5 hours to the closest Apple retail store they'd probably figure it out and fix it for free.

Anyways, he said he wouldn't know till at least monday, so when I find out I'll post the results.

took is to the Apple store a few hours away, they replaced the logic board and now it's fine.