FreeBSD 5.3 forgets some of my users

Hello there,

there is a strange thing.... FreeBSD 5.3 randomly frogets some of my 
users. Sometimes I see in the first column of the ps aux output the uid 
number instead of the login name, but next when I run it, there is the 
login name. Besides, I tried to chown a directory to an existing user, 
but the I get an error message, that there wasn't such user.
I immediately checked passwd, group and master.passwd files in /etc but 
the entry for that user was present there. The pw userdel was unable to 
delete that user, so I had to manually remove it from those three files 
and create it again. It worked then, but a bit later there was the same 
result. I'm quite annoyed now. This state isn't safe enough, I have to 
do something to get around with this. Do You have similar experiences? 
Or do You now some kinda workaround?

Cheers,

Gábor Kövesdán