Also you could try verifying the disk.

Open Disk Utility. Click on MAC in the sidebar. Then click Verify Disk. If there are any errors then click Repair Disk.

Then you might want to also try clicking Verify Disk Permissions and then Repair Disk Permissions

If after doing all this and there is no improvement try backing up your folders and files and doing a clean install... in other-words completely erasing the whole hard drive and re-installing the operating system from the instal disk.

Also, you could try verifying the disk.

Open Disk Utility. Click on Macintosh HD in the sidebar. Then click Verify Disk. If there are any errors then click Repair Disk.

Then you might want to also try clicking Verify Disk Permissions and then Repair Disk Permissions

If after doing all this and there is no improvement try backing up your folders and files and doing a clean install. In other-words, completely erasing the whole hard drive and re-installing the operating system from the install disk.