I have an early 2008 Mac Pro, which has re-booting problems. Also what does the spinning beach-ball indicate?

Question

Hi, I have an early 2008 Mac Pro which has re-booting problems.
Processor speed is: 2.8
Memory: 2GB 800 MHz DDR2 FB-DIMM
2 x 28GHz Quad Core Intel Xeon
I am running OSX Yosemite Version 10.10
My Mac Pro keeps re-booting. Last year I had to replace my graphics card. My original card was the ATI Radeon HD 2600 XT 255MB, and that is what I have now. At this precise moment my Mac Pro is running perfectly, except that it is slow and the spinning beach-ball keeps appearing. I have managed to do some work with the disk utilities, verifying, cleaning and partitioning. Some errors were found and when it was cleaned this seemed to help my Mac Pro to function properly. Although I am able to use my Mac Pro now, from day to day I still experience re-boot problems. Also quite unexpectedly my mac dictionary has an error, it closed itself down and will not open at all, I had the message to say that a report will be sent to Apple.
I have tried starting my computer with an external hard drive fitted via a USB cable, I use for back-ups. This worked and I was able to wipe my hard drive clear and replace all info from the back up I had done only a few days ago.
This worked for a few days and then the same problem started again.
I am beginning to wonder if I need to buy a new hard drive.
If there is anyone who has some answers to help me solve my problem, I would be most grateful.
Robert

Answer 1

When you have the beachball activity, note the exact time: hour, minute, second.  
These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.
Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad and start typing the name.
The title of the Console window should be All Messages. If it isn't, select
          SYSTEM LOG QUERIES ▹ All Messages
from the log list on the left. If you don't see that list, select
          View ▹ Show Log List
from the menu bar at the top of the screen.
Each message in the log begins with the date and time when it was entered. Scroll back to the time you noted above.
Select the messages entered from then until the end of the episode, or until they start to repeat, whichever comes first.
Copy the messages to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.
The log contains a vast amount of information, almost all of it useless for solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.
Please don't indiscriminately dump thousands of lines from the log into this discussion.
Please don't post screenshots of log messages—post the text.
Some private information, such as your name, may appear in the log. Anonymize before posting.