is DNS reliable ?

[Dariusz Zmokly]

hi !

I have dns server with some virtual domains. Sometimes it is impossible
to get dns answer for some zone from outside. So I wrote a script 
doing nslookup to six primary servers for Poland (pl.). This script
was run hourly and tried to get addresses of all my domains from these
dns servers. 

Well, I got strange results. 3 of 6 servers works as I thought they would
work - show the same results as my local dns. 

But another 3 servers works more or less stochastic. Say, at 21:00 
server A says "can't find www.duli.pl: Non-existent host/domain"

After an hour I got another answer and this time that server gives correct
output. 

So I wonder if negative answers could be not reliable ? If server can't
find information about particular domain in some amount of time would
it send answer that this domain doesn't exist ? Or mayby I have some
misconfiguration of dns here ? 

best regards,
DAriusz Zmokly