Well Dave you best bet will be to take it to the nearest appleStore or call AppleCare if you got it. By the way IMHO the best OS X disk maintenance program on the Mac is Alsoft's DiskWarrior.
If you still have the original OS X install disk one of them have a Hardware Test that is a bootable DVD on one of the disks to run a test on the Mac system. See if you run that to see what it says.
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