Mac Pro self-restarting

Question

About the computer: early 2008 Mac Pro dual 2.8, 4GB stock ram, OS 10.6.7. Additional hardware includes: aftermarket ATI HD3870 (to replace failed stock card), Black Magic Decklink I/O; 1TB OSX HDD, 320GB Windows 7 HDD and 2x 1.5TB storage drives in RAID 0; Pioneer BD burner.
When booting from cold (an recently when waking from sleep) computer self-restarts at any point from powering until running about 10 minutes. Once the computer has been on from more than 10 minutes restarting does not occur, expect when waking from extended sleep. Typically, when booting from cold, the computer will boot full and the OS (OSX or Windows) will run for several minute. The computer then restarts. Following the first restart, subsequent restarts generally occur before the OS is fully loaded, but the exact timing varies (almost always following the "chime"). The process continues about 10 minutes or so until eventually the computer boots fully and stays on. Incidentally, occasionally--but not typically--the fans kick on high when starting the computer cold.
My suspicion is this is indicative of a power supply starting to fail. But I'd like to rule out other, more easily remedied  possibilities first. So far I have tried resetting SMU and PRAM/NVRAM and as well as booting with the windows hard drive and blueray drive removed without success. I set the computer to boot in Verbose but cannot see any indication of what's wrong; previous shutdown cause reporting "0".
Any suggestion--even  a way to test the power supply--greatly appreciated. Thanks very much in advance.

Answer 1

I have a Mac Pro 1,1 that recent gave me similar problems.
When in Leopard or Windows XP, computer would randomly shutdown completely. Typically I was surfing the web and seemed to happen more often when watch Youtube videos.
Started off happening once in a while and then computer was unusable as it would not complete the boot up in to either OS. When I would power up after a shutdown, light would come on and I would hear "TaDa" and could hear drive activity----then sometimes it would just cycle with repeat attempts as noted by another "TaDa". If I was really patient, it would sometime boot into stable configuration only to have computer just have another hard shutdown after a few minutes.
Did a lot of internet searching and I though it was the logic board or power supply. Did SMC rest, PRAM reset. Checked the DIAG_LEDS. Figured it hard to be a hardware problem as issue was present in Mac OSX and XP.
Ultimately caved in and took computer to the Apple Store---they ran diagnostics. Diagnostics said one memory riser card was failing---replaced it for $50. Got home---same problem.
Back to Apple Store for a week. They replaced the second memory riser card and all is better now. Was not logic board or PS.
Computer is fine except can no longer get the Startup Manager screen to come up after holding OPTION key to select which OS (Leopard or XP) to boot into.
Hence the reason I came to the Apple Discussion board. Saw your problem and though I would add my recent experience.
I will be adding my current issue under a new thread.
You might try seeing if the problem occurs in OSX and Windows---if so---seems to indicate it is hardware and not software.
Good Luck.
Good Luck