NT4.0 Server re-boot by itself

I have an NT4.0 Server with sp5, 128mb of Ram,8G of SCSI HD, SCSI tape backup, IDE CDROM, 1.44 fp. Server will reboot (not every time)when I insert the tape to do a backup or when printing a document. I replaced the power supply/ethernet/cdrom but still have the same problem. It seem to be working if leave it alone like no touching.