Debian :: Installed With Debian 3.1 - Server Restarts On His Own

I have a problem with one a server installed with Debian 3.1, that is restarting almost every day at the same hour, 6,25 AM. Here are the messages obtained with #cerberus:/etc# grep -C 5 restart /var/log/messages

.....missing displays....

.....missing displays....

.....missing displays....

As I said, there are days when the system is not restarting, but more often it does. For example, on 6th and 8th October it didn't restart, but on 7th it did, and the messages are the same.