odd problems

[Tirips Beta]

bind 8.2.4, redhat linux 2.2.16
using quickdns to maintain the data files.

sometimes our mail server stops responding, which isn't a dns issue,
connections time out from outlook express, well, since the linux dns box is
the new guy in the server room, he is the first suspect.

-they can still ping the mail server and its getting the correct ip
address, so its resolving fine
-they can use the web mail interface of the mail server, so its resolving
fine
-they get server timeout errors using OE, so its resolving fine, just not
connecting to the mail server.

(why they thought of doing these following things is beond me, but from
what i understand of networking it shouldn't matter, but for some reason it
does)
-they switch there primary/secondary dns servers so they aren't using the
linux machine, and oe starts to work again, so its the dns server.

-they enter in the ip address rather then the host name and it works, so
its a dns issue.

anyone have any ideas of what i should be looking for so i can get the
finger of blame pointing the right way? or maybe what is going on?