>On one of those strange days, a server decides to reboot 4 times in a row by only keeping itself alive about 20 seconds at the login prompt.
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>This Windows NT Server only created one log in the Event Log, at each occasion, which says something about an unexpected shutdown. That's it. That's all. Where else can I go to extract additional information regarding those unexpected reboots?

As from one who was downtime when that happened, it seems it might have been caused from an electric thunderstorm which last for a few minutes. The power system had suffered a little in that sector. However, we have a good UPS. Could it be that an electric thunderstorm might have bypassed the UPS?