NT Server reboots

Recently I have been facing this problem quite often. What happens is my Hp Net LDPro server reboots on its own. When I checked the event viewver it shows that the shutdown was unexpected. What causing this? The server already gone through a maintenace service about 2 months ago. After the service it was OK. But now it started again. Could it be related to power. But the odd thing is it reboots after office hours. And once in a while during office hours.
The server specification is as follows:
model: HP Net Server LD PRO
processor: Itel Pentium Pro 200Mhz
Ram : 128
HD: 6GB (SCSI hot swappable)
OS: Windows Nt Sever4.0 (SP 5)

Please help.
Thank You.