PC restarting randomly

I have a Dell Dimension(maybe 2 years old) on my network that will periodically restart randomly. It does this even when disconnected from the network. It is running windows98. Every time it reboots, I get lost clusters. I have used Norton Utilities 2000 on it and ran a full disk doctor and speed disk. No damaged areas on the hard drive were found, so I do not think it is a bad hard drive. It will reboot even if it is just sitting there, does not matter if I am in any programs. I have already formatted the hard drive completely and reinstalled everything, and it still does this. This started a couple of months ago, but has been more frequent lately. Does anybody have any suggestions? I’m wondering if it is hardware?