Nagios Crash


I have a problems with nagios.

After start it works fine about 2 hours.
After that it stops all checks.
Then I running
ps ax | grep nagios

I see that:
19871 ? Sl 0:49 /usr/local/nagios/bin/nagios /etc/nagios/nagios.cfg
7295 ? S 0:00 /usr/local/nagios/bin/nagios /etc/nagios/nagios.cfg
7296 ? Z 0:00 [nagios]
7903 pts/1 S+ 0:00 grep nagios

After restarting nagios it works fine but the problem appears again.

Nagios Version 3.0.5

Ubuntu Linux
kernel 2.6.18

P.S. Sorry for my english