Exchange server 2003 hangs up

Hi everybody,

E-mails are still normally delivered, but I am unable:
- to open a session on the server.
- to remotely connect to the server via a computer management mmc snap-in
- to access server's shares

The only way to restart the server is to power it down and on.

Just after restart everything seems to work BUT after 5~10 minutes the I get the same hang up behaviour.

I see the following in the event log:

Event Type:    Error
Event Source:    MSExchangeSA
Event Category:    (1)
Event ID:    1031
Date:        04/05/2009
Time:        14:25:24
User:        N/A
Computer:    OCT-EXBE01
Description:
One of the System Attendant's task is blocked.
Function: CMonitoringTask::Work.


Could anybody help, please?