psa service will stop by oneself?

[samyu]

Psa service will stop by oneself , must carry out psa start manually , will happen once in about one day

/etc/init.d/psa start
Starting psa-spamassassin service: [ OK ]
Processing config directory: /usr/local/psa/admin/conf/httpsd.*.include
Processing config file: /usr/local/psa/admin/conf/httpsd.expand.include
/usr/local/psa/admin/bin/httpsdctl start: httpd started
Starting Plesk: [ OK ]
Starting up DrWeb(R) daemon: [ OK ]

When stop, will receive it by the information that server will be sent out , the following

Hello, Default Administrator
Plesk server xxx.xxx.xxx.xxx changed its network status to "offline"

anyone help me? thanks