sporadical login failed?

Hi,

I am runinng courier imap (4.0.6) authentificating against mysql.
For a short time i am receiving sporadical "login failed".
I seems as if Courier isn't able to "contact" authdaemond.
(if the login failed no query is submited to mysql)

This seems to be a resource related problem.
Any hints where to look at?
memory/cpu and disk load seem to be within normal parameter.
Could this problem be related to any system limit (max sockets etc?)

Cheers,
Nikolas