Thank you for your reply. I'm going to have you do a few things to maintain your Mac and fix the issue.
1) You're OS X is out of date. Please run Software Updates and install the updates.
2) Repair Disk Permissions on your Mac:
1. Log in to your computer with the administrative account. Open the hard drive, go into the "Applications" folder and then open the "Utilities" folder. Now launch the Disc Utility application.
2. Select your main hard drive. You'll probably see two options--one is the manufacturer's name of the disc and the other name is XXXXX XXXXX've called it. Choose the name of your hard drive.
3. Click on "Verify Disc Permissions" and the computer will perform a diagnostic test. After a few minutes, the problems with disk permissions will be reported in the Disc Utility window. Click on "Repair Disc Permissions" to fix them.
4. Insert your Mac OS X installation disk and restart your computer. Hold down the "C" key on your keyboard to boot from the disk rather than your computer's hard drive. Cancel the OS installation and open Disc Utility.
5. Click on "Verify" to have Disc Utility check your computer's hardware. Notice that this option is only available when the program is run from the CD. You can only test the hardware of a computer that isn't running the program locally.
6. Use Disc Utility to try and repair any hardware errors that it detects. Usually fixing the permissions will be enough to maintain your computer. If you still experience frequent lockups and crashes, then you should try repairing it with this method.
3) Reset PRAM and NVRAM:
-Shut down the computer.
-Locate the following keys on the keyboard: Command, Option, P, and R. You will need to hold these keys down simultaneously.
-Turn on the computer.
-Press and hold the Command-Option-P-R keys. You must press this key combination before the gray screen appears.
-Hold the keys down until the computer restarts and you hear the startup sound for the 2nd time.
-Release the keys and let your system boot up.
4) Reset SMC of an iMac:
* Shut down your Mac
* Disconnect the power cord
* Press and hold the Mac’s power button for 5 seconds
* Release the button
* Reattach the power and boot the Mac as usual