It looks like you've tried the usual software troubleshooting. Perhaps you have a hardware issue (failing motherboard, hard drive, etc.) One of the Forum experts prepared this troubleshooting tutorial which includes various hardware tests in the first "Information" box. I'd start with them. If hardware checks out OK I'd try one more time with the actual troubleshooting steps in the order listed. Follow the steps exactly and don't skip any steps just because you might have tried it previously.
