Mac Pro suffering from kernel panics almost daily
In my mind there are a couple of likely issues.  If the machine is crashing with a kernel panic I would suspect the hard drive is on it's way out.  If the machine is simply restarting without warning then I would suspect the memory.  

To see if it is the memory it might be as simple as removing each stick of ram and blowing any dust off with compressed air and putting it back in.  If this doesn't resolve it, I would get a new drive and clone the system using a tool like Carbon Copy Cloner or SuperDuper!  Boot from the new drive and see if the issues go away.  

If neither work then you could try reinstalling OS X, I doubt that's the issue.  You more than likely have a failing video card or logic board if the above suggestions don't work.