I have a G4 dual 500 MHz, running 10.4, that suddenly won't complete the start up sequence. When I press the power button, I hear the chime, the monitor wakes up, then the Mac shuts back down. It doesn't make it to the gray Apple logo. Here is what I have already tried: I've tried booting to a system CD or DVD by holding down C. No luck. I've tried getting the startup disk menu by holding down Option. No luck. I've tried every keyboard shortcut I can think of. The only key combination that works is command+option+P+R. This seems to reset the PRAM (I hear the chime again, repeatedly if I hold it down) but it doesn't fix the problem. I've tried holding in the power button continuously to boot into Open Firmware. I hear the flat beeping tone, but it still shuts down before anything shows up on the monitor. I have access to another G4 of the same make and I have swapped out all the parts I can think of, using parts that I know work, including the RAM, hard drive, optical drive and video card. I replaced the power supply about 6 months ago. I haven't tried swapping that because it does START to power up. I assume that if the power supply had gone bad again, it wouldn't do anything at all. Is that a false assumption? There were no warning signs that I can think of. One day it worked fine. The next, it didn't work at all. I am completely flummoxed and out of ideas. HELP!