The problem: don't wakeup after 2 or 3 hours, don't boot if the computer was shutdown for 4 hours or more.
The startup begin. I can hear the fans startup but the screen remains turned off.
The cap led never turn on. There is no tones or startup chime. I do not install any extra card, ram, etc.
After a restart (or reboot) the G5 work perfectly.
What was done to fix the problem:
reset the nvram
reset the pram
reset the motherboard (PMU reset button)
remove and replace the video card (ATI 9600 xt)
exchange the two DIMM (two 512mg -> 1 gig ram)
unplug every external connector (video cable, mouse, keyboard, ethernet, etc) unplug internal hard disk, boot from external firewire disk (like the internal drive the firewire disk boot correctly the second time) replace the 3.6v battery.
Sometime the G5 boot the first time if the Apple Hardware Test CD-ROM is in the SuperDrive.
Is there anything I can try before returning the G5 to an Apple Service store ?