Google webmaster Tools - DNS lookup timeout

URL timeout DNS lookup timeout

[skfree]

I have used Google webmaster tools to monitor and check our domains. Problem is, that google signalled on pair following error:

URL timeout: DNS lookup timeout
We encountered an error while trying to access your Sitemap. Please ensure your Sitemap follows our guidelines and can be accessed at the location you provided and then resubmit

Resubmit of sitemap does not fix problem, some of domains are working correctly, some dont. I am suspicious, that there is some problem for GOOGLE with our DNS bind9 powered server, but how to track it down? Domains are working normally, DNS check page does not show anything interesting.

Working domains (currently)

skfree dot net
emed dot sk

Not working (timeout)

skfree dot sk
magister dot sk

Strange is, that they have SAME zone file.

Please, do you know, how to check why google is getting DNS timeout?
I go thru webpages on google about this problem, but no interesting things...

thank you
Milan