DNS problems?

[craig baker]

I setup a Centos 5.1 perfect server according to directions. postfix bind9 etc.
I'm getting reports that occasionally my users are getting DNS errors when trying to access the server. urls not found etc etc.
its from more than one person in more than one area (or I would suspect a corrupted DNS cache or pilot error).
now these complaints are not hourly or daily, but several times over a few weeks I had one fellow not be able to upload files since it said it could not find the server, and another say his listserv emails were not getting through - and talking to the listserv maintainer they say they are getting occasional DNS errors resolving my url when they want to send me a batch of email

how can I stress-test a DNS server and see if its performing well enough???

the dns server is ns5.cdbsystems.com

any help would be appreciated.

cdb.