Four times out of five, on average, the Mac boots to a black screen -- a 30" Cinema Display.
I believe the computer is booting all the way up, because I have the front power button set to put the computer to sleep, and when I manually power down the machine using the power button, the Mac will go to sleep first.
Also, the Mac will go into Firewire disk mode when connected to my MacBook.
The "boot-to-black-screen" occurs from cold or warm boots.
I've tried both 10.5 and 10.6 OS -- no difference, results the same on both.
Here's what I've tried:
Reinstalling and updating OS both 10.5 and 10.6
Pulling all USB and firewire peripherals off the computer (except the Apple Wired Keyboard that shipped with the PC) -- Mac promptly booted to black screen, so peripherals are not the issue.
Changing displays (in this case, a Samsung 17") -- same result
I have tried booting to the Apple Hardware Test on my commercial Leopard DVD and Snow Leopard DVD, but the machine will not boot to this (the Leopard disk will boot the AHT on other Macs). I no longer have the original CDs that shipped with the Mac.
I've suspected the video card, but according to other posts, apparently this is not the cause.
I'm now suspecting a motherboard firmware issue.
Before this, the Mac Pro was having a related issue pertaining to VMWare Fusion, wherein the Mac would occasionally not wake from sleep, logging a VMWare Fusion kernel crash.
For the moment, I'm not shutting down the Mac Pro, just turning off the Cinema Display. I've left this computer running for up to two months without a reboot, so I'm confident that it will stay up and remain stable.