So I've had my Mac Pro since 2008. I currently have OS 10.6.8 on it. In the last week, it has restarted itself twice while I was in the other room. Since I wasn't there, I don't know if there were any messages. This has never happened before.
When this happens, it takes quite a few hours before it will operate normally without grinding the hard disk for a minute or more at a time. Most applications freeze or become very unresponsive while it does this.
The disk is a 320 GB drive, and there are 42 GB free still. I've done a DiskUtility Verify Disk on it, and it says everything looks normal.
Is this some issue that has a common cause, or do I need to take it in to the shop?