Unscheduled shutdown on NT server

I have an NT server that has started rebooting every day betweeen 5.30 and 6 am. Nothing in error log except ftdisk recovering from dirty and event log reporting that the shutdown was unscheduled. This has happened every morning for the past 2 weeks, before that never. So far as we can work out nothing has been changed in the configuration over that period. I have seen similar before with RAM/cache faults but then there were stop codes to point to the problem. Any ideas would be welcome.