G4 PCI & AGP Graphics Shuts Down by itself



I just put a new DVD ROM and 120 Gb HD in my G4. I was using the G4 this weekend and I usually just leave it with a screen saver going. I noticed about 2 hours later it was shut down and it would not come back up, after alot of troubleshooting I changed the battery and now it starts up, It was like the plug was out that is how dead it was. Now that it is up and running, It has shut down on me twice approx a half hour into working on it. I have to reinstall my Quark activation everytime it does this, because it comes up asking to fix the clock. I am not hooked up to the internet this is just a work machine. Does anybody have any idea why it keeps shutting down? It is Not the Powersave. Also the light flickers but no chimes when I boot up.

Sorry for Rambling but I wanted to give all the info I could.

Please help