just updated to 10.8.3 and my system has crashed several times

You have a Mac Pro. PowerMac went out 7 yrs ago and can't run 10.7 or .8

You did have Lion? or you had 10.8.2? which is it?

When you went from 10.6.8 to 10.7 or above, was it over the old, or new fresh clean system install? most people may not know that they can and should do clean install and that it is more stable reliable and does not take longer - often faster. Plus that gives you dual boot to continue to boot the old OS (10.6 or .7).

CLONE your system before you do any type of changes.

At this point with the crashes lets assume you need to repair the drive from ML Recovery,

Recovery Mode

http://support.apple.com/kb/HT4718

insure you have backups (clone AND TimeMachine) and that the directory is dirty, that you probably have some driver extensions that get in the way. Even preferences and cache files.

What apps are running at the time?

What graphic card do you have? and which model year? 3,1? 5,1?

Format another drive and install just Apple Mac OS and updates.

Some point a log of the crash highlights and hardware overview (sans serial number)

Using Cloning as a Backup Strategy

http://www.apple.com/support/lion/installrecovery/

Create an OS X Lion Install disc

http://reviews.cnet.com/8301-13727_7-20080989-263/how-to-create-an-os-x-lion-ins tallation-disc

How to clone your system:

http://macperformanceguide.com/Mac-HowToClone-backup.html

http://macperformanceguide.com/Mac-HowToClone.html

http://www.macupdate.com/app/mac/7032/carbon-copy-cloner

Post the panic log. Here is how to get it:

Mac OS X: How to log a kernel panic

Cut and Paste the text into a forum window, then before you click "Post message" you may want to remove serial numbers and personal information.

In an old post you left it as is but dealt with how Lion dropped support for PowerPC and of course that is still true, and any OS upgrade means making sure everything you run and use or that is installed is compatible and supported.

http://roaringapps.com/apps:table