Hi,
I am using Hotspot for several years, my settings of MT DNS were pointing to primary DNS on W2000 Server and secondary DNS on BIND on our ISP machine. Now I changed primary DNS to W2003 server and secondary to new machine of ISP (also BIND). From that time I have huge problem with DNS. Symptoms are very close to this discussion:
http://forum.mikrotik.com/t/dns-problem/15495/1
I cannot find any reason, but starting to be nervous. Any suggestions?
Thanks.
it would be nice if you wrote what exactly you are seeing, so we don’t have to jump between discussions and guess what is almost similar in your case 
OK, try to describe it:
When client is logging in through hotspot, everything works fine, pings are fine, in the begining Firefox or other browser is displaying webpages correctly as well. Lets say as example, that http://www.google.com is my homepage. I see this page, I see all pages, but suddenly, sometimes after 5 min. sometimes after 10 min., sometimes immediately I can see errorr page wit DNS error message (for many pages, not only http://www.google.com). Certainly ping and other direct IP connections to internet are still working fine. When I use nslookup at this moment and asking for this address, I am receiving error message “no A record for this addresss…”. If I try to ask nslookup within another 5-10 min., it is answering correctly, but in a browser still the same DNS error.
For some users I am using IP binding and bypassing hotspot, for these users everything works fine except users with Apple computers (these have DNS problems).
Another important fact is, that domains, which have records on my primary DNS server are working correctly anytime.
Maybe one more detail:
When I start computer, everything works, within some time DNS stops working.
In nslookup I tried ie. query to http://www.hotmail.com
Answer was “no A record for http://www.hotmail.com”
Tried again within a few seconds and there was correct answer. But in browser still DNS error. Restarting browser, cleaning DNS cache, nothing … Still does not work. Flushing cache in Mikrotik, nothing.
After computer reboot some pages work, some not. Good example is:
After reboot, http://www.google.com works, http://www.gmail.com works , but maps.google.com does not work. Within 5-15 min. nothing works.
Looks I will answer myself. My provider gave me a wrong nameserver address we can use. There were settings, which kicked us out after several queries 
((((((
i have had this days some problems also with dns , give a try opendns.com