unexpected re-boots

I have an account that uses Small Business Server with about 15 workstation clients. The only app running is Peachtree Accounting. Modem share is used for the internet, but there is no internal e-mail. Approx. 4 days a week, this server will reboot on its own. There is nothing in the event log to suggest what is causing this. One time I was standing right by the system when it did the re-boot and I know it was not a power problem. The server has a UPS connected and there was no indication of a power outage. It’s as if someone directed it to shutdown and reboot. It’s getting worse each week. Any ideas?