DNS interruptions

Hi,

I have a problem with the DNS server on our Mikrotik router (ros4.16) and Apple clients (OSX 10.6).

At times, the clients can not resolve DNS names.
A DNS cache flush on the clients fixes the problem.
After some time the problem occurs again.

Does anyone have any idea what may be the cause of the problem and how can it be fixed?

I’ve never been able to fix it, and just use other DNS servers.