For a starter you can read this article that explains some of the reasons. It could be but I personally do not think it is a hard drive problem. That would be low on my list of posibilities. You have not given much information to go by. You could help by reviewing the event logs, you did not tell us if you can ping the server, you did not tell us in you can do remote management on the server during these times, and you did not tell us if you can remote in from a workstation. You did not tell us the role of this server, is it a DC, file server, Exchange server, member server etc. We really need more information.