I"m no Mac expert, so the best I can give you is maybe. Try running Apple Hardware Test if you can. On Reboot, hold down Command R (I think) which should get you to the all-in-one helper (sorry, forgot name). there you should be able to choose which hard drive to start up from and then do a repair on it. It's not normal, that's for sure..
It could be the logic board,video card or hard drive. the Radeon X1900 is known for clogging up with dust and giving up the ghost a bit early. try shutting down your Mac Pro, letting it cool, take off the side panel, grab a can of compressed air and go to town on that video card. You may find a whole bunch of dust bunnies in there. that being said, after doing that, putting the side panel back and rebooting, the SMC may need to be reset. If that doesn't work, it could be the logic board, so you'd have to take it to your local authorized Apple repair place. Oh, I"m not totally sure if this is still good, but there might still be a recall on the X1900 video card. Check to see if your video card is one of those affected ones. Just a thought. If you can, a new ATI 5770 video card is around $250 in price, not too hard to install and will be better than the X1900, if you can afford it.....
You know all the issues there are with that ATI X1900 and dust and heat right?
Also about heat and cooling FBDIMMs where extra care and increased fan speed is a must?
I would of course continue with Safe Boot, Recovery Mode, and to also clone your system for bootable backup with Carbon Copy Cloner.,
Might want to get the ATI 5770.
More RAM definitely.
Proper cleaning of dust bunnies.
ATI Radeon 5770http://www.amazon.com/Apple-ATI-Radeon-5770-MC742ZM/dp/B003Z6QH6
How To Install and Remove Memory Mac Pro2x2GB FBDIMM DDR2 667MHz @ $35
Metro Vacuum ED500 DataVac 500-Watt 0.75-HP Electric Duster 120-Volt
How to clone your system:http://www.macperformanceguide.com/blog/2012/20120711_2-MacPro-internal-clone-ba ckup.html
General purpose Mac troubleshooting guide: Isolating issues in Mac OS Xhttp://support.apple.com/kb/TS1388
Creating a temporary user to isolate user-specific problems: Isolating an issue by using another user account
Identifying resource hogs and other tips: Using Activity Monitor to read System Memory and determine how much RAM is being used
Starting the computer in "safe mode": Mac OS X: What is Safe Boot, Safe Mode?
To identify potential hardware problems: Apple Hardware Test
General Mac maintenance: Tips to keep your Mac in top form