The symptom you are seeing where the whole screen goes black w/ white text telling you to restart sounds like a Kernel Panic. It occurs when there is a software malfunction or glitch in the system so severe, it does not know how to proceed and pretty much goes into shutdown mode.
These can be caused by a number of things including corrupted software, external devices, or even hard drive and logic board issues.
The multiple restart issue when you try to boot back up from the Kernel Panic sounds suspicious, but this is the best way to isolate things down:
I would start by removing all of those external and USB devices except the mouse and keyboard and see if the issue still occurs without those. If it does not, but then you plug them in and the issue comes back, you've isolated it down to one of those devices.
If the issue STILL occurs after you remove the devices, you will want to do some OS troubleshooting. Find the install disc for the Mac, and follow this article to boot to the disc to run a
Repair Disk on the Macintosh HD.
http://docs.info.apple.com/article.h...n/duh1018.html
After you do this, Quit the Install Disc and boot back to the system and test again. See if this helped resolve the issue or if it still occurs.
If the issue STILL occurs, you will want to perform an
Archive & Install Preserving Users, as described here:
http://support.apple.com/kb/HT1544
This reinstallation will NOT remove your Programs or Data; it will simply reinstall OS X around all of it and will more than likely resolve the issue. After this is done, you will need to perform your Software Updates again, and there will be a Previous Systems folder on your Desktop you can eventually trash.
If that DOESN'T fix things, get back with a reply...because beyond that, you are looking at backing up and Erase & Installing, performing an Archive & Install Not Preserving, or having to bring the unit in for service.