Run the Apple Hardware Test:
http://support.apple.com/kb/HT1509
Also, have you checked the boot disk for errors? Disk Utility might be able to fix it.
Open Disk Utility, in the Utilities folder in the Applications folder.
Select your disk, and click First Aid.
Click Verify Disk.
If the disk needs repairs, do one of the following:
Insert your Mac OS X Install disc in your computer’s optical drive or an optical drive connected to your computer, and then restart your computer. When you hear the startup tone, hold down the C key until you see the progress indicator, which looks like a spinning gear.
Follow the onscreen instructions until the menu bar appears with the Utilities menu in it, choose Utilities > Disk Utility, click First Aid, and then click Repair Disk.
Restart your computer using Remote Install Mac OS X. Follow the onscreen instructions until the menu bar appears with the Utilities menu in it, choose Utilities > Disk Utility, click First Aid, and then click Repair Disk.
*Checking a hard disk to see if it’s going to fail*
You can check the health of your hard disk by checking its Self-Monitoring, Analysis, and Reporting Technology (S.M.A.R.T.) status in Disk Utility. Most disk drives include S.M.A.R.T. hardware. Developed by IBM, S.M.A.R.T. hardware allows hard disks to test themselves for potential problems.
Tip -- Run Disk Utility and Repair Disk Permissions before and after all software updates.
Launch Disk Utility. (Applications/Utilities) Select MacintoshHD in the panel on the left, select FirstAid, then click: Repair Disk Permissions. Quit DU when it's finished and reboot.
If necessary, if you have another Mac, you can use Target Disk Mode to transfer files. You need a Firewire cable in order to do this.
http://support.apple.com/kb/HT1661
Carolyn 🙂