Dear support community:
Why is my Mac Pro restarting itself?
Dual Quad core 2.8 GHz, OSX 10.6.8, 10 Gigs of RAM
PCI card installed: Universal Audio UAD-2
It seems to exhibit these general behaviors:
It boots up successfully, acts perfectly fine for 8 minutes
and then restarts istelf.
Here is where the situation alternates a bit
1. It locks while rebooting, the power button light flashes slowly, the screen is black
2. It locks while rebooting, the screen shows a 'boot in progress' bar across the bottom
3. It reboots and is fine for the rest of the day.
4. Kernel panic screen while rebooting
If case 1 or 2 happen, I may have to interrupt and reboot one or two more times.
I have repaired permissions several times, booted from the OSX installer disk,
run disk utility and TechTool Pro a few times as well.
your video card could be shot. i've already gone through two videos cards in my mac pro over the past 4 years. my mac started to exhibit a lot of the same symptoms you mentioned each time the video card started to go.