FreeBSD-7 reboots hourly

[Alexey A. Ukhov]

Hello dear colleagues.

2 days I have strange behaviour on my 7.0-RELEASE-p3 server.
It started to reboot once an hour.
No any suspicious task in crontab, nothing strange in /var/log.
Just silent reboot.
I have no physical access to server - only remote one.

Question is: what could it be?
How to find what send computer to reboot?

Just extra information: I had such stuff before - installworld helped me 
for one week,
now it appears again :(

Opened services/sockets are:
nobody   proftpd    647   1  tcp4   *:21                  *:*
root     sshd       845   4  tcp4   *:22                  *:*
postfix  smtpd      5115  6  tcp4   *:25                  *:*
bind     named      525   20 udp4   xxx.xxx.xxx.xxx:53    *:*
nobody   nginx      643   4  tcp4   *:80                  *:*
root     couriertcp 769   3  tcp4   *:110                 *:*
root     couriertcp 789   3  tcp4   *:143                 *:*
root     rsync      659   5  tcp4   *:873                 *:*
root     couriertcp 779   3  tcp4   *:993                 *:*
root     couriertcp 760   3  tcp4   *:995                 *:*
root     syslogd    461   9  udp4   *:514                 *:*

also on local interface (lo0) I have apache, postgrey, spamassasin.

I need some pieces of advice what to do with it and what could it be.

Thanks in advance,
Alexey A. Ukhov

P.S.: I mean hourly: I start top and just wait. last what I see is: 
0:59:59 uptime