System rebooting

[Bob]

I am using Ubuntu v14.04.  The computer is currently being used as a file and
print server.

The system is rebooting at random times for some reason.  I looked in the
SYSLOG but do not see anything that indicates the reason.  Is there some place
that would indicate what the reason for the reboot is?

In the SYSLOG I see a long string of ^@ characters at the time of the reboot. 
What do these characters mean?

-- 
Robert Blair