I personally had a similar situation. We had an accounting firm whose servers we were maintaining, and occasionally the print server would reboot, always between 9 and 10 am. So I sat in there for a week between those hours and noticed the light would dim and occasionally the reboot would happen.
It turned out that the lawyer next door would come in, turn on his PC, printer and coffee pot simultaneously because they were all on the same power strip, and the drain was causing an undervoltage on the circuit the server was on during startup. We had it on a UPS, but it turns out that at the time consumer grade UPS systems only handled outages.
I measured drops as low as to 85 volts, in practice anything under 95 or sou would reboot.
It turned out that the lawyer next door would come in, turn on his PC, printer and coffee pot simultaneously because they were all on the same power strip, and the drain was causing an undervoltage on the circuit the server was on during startup. We had it on a UPS, but it turns out that at the time consumer grade UPS systems only handled outages.
I measured drops as low as to 85 volts, in practice anything under 95 or sou would reboot.