Weird Reboot issue

I have three servers in my organization sperated by a Frame relay WAN connection.

The first server, our PDC, is a NT 4.0 with SP6a installed. The next two are Windows 2000 servers.

My issue is that the servers, all at once, reboot. Literally, they all reboot at precisely the same time.

I have run all the current virus definitions at the servers, and nothing is coming up.

What could be causing this very frustrating issue?

Thanks.

JB