The house was rewired about 8 months ago. There have been no power problems. The computer is currently plugged directly into the wall.
I managed to get the thing started earlier this evening and switched the startup disk. It started up a number of times consecutively, but a few hours later, nothing.
1. Buy a UPS. Never plug your G5 directly to AC power. In addition to protecting your computer from power events, you may learn something about the quality of your local power grid. When I lived in Southern California (1997-2005) I had constant power problems. The UPS solves all of them. There are many brands on the market. I like APC
. It needs to be fairly large for a Power Mac G5. Mine is 800VA.
2. Remove and re-seat every card (Video, RAM, etc.) in your computer. Corrosion of the edge connectors on plug-in components can cause intermittent problems that are nearly impossible to diagnose. Do the same with your cable connections on the back of the computer. Unplug, wiggle in and out, replug. When you are done with this, reset the SMU and PRAM.
3. Boot from the OS X DVD.
4. Post back and tell us what happens.
And more questions:
1. What is the model number of your G5?
2. What make/model of display are you using?