I found the problem of my servers, and this could be the problems for your as well.
most of my server are busy, running lots of processes and nagios server EXTERNAL COMMAND CHECK INTERVAL was often as possible by default causing it to stop or no to be able to process the result back to the server, and as soon reached the TIME OUT value produced "Plugin timed out while executing system call"
i change to be at least 3minutes or 180s
in the nagios.cfg
and restart the service, until now i haven't have any of those problems anymore.
also you should check # TIMEOUT VALUES section.
i hope this help some one.