DNS problems

I have a Win2k DNS server on a network with aproximately
20 PC clients some of them running Windows98 and the
others running windows 2000 pro.
I have a Dial-Up router as my default gateway to the
internet.
The problem is that my DNS connects to the ISP every 5
minutes and stays connected during 1m:40s and then
disconnects himself. I think that the problem has
something to do with primary to secondary SOA record
transfers. This problem occurs even if no one is
connected to theinternet at that time.
I had the same problem in a NT 4.0 server but i solved it
by applying the service pack 6.0. Microsoft has declared
that in Windows NT 4.0 this was a know bug.

Is it the same with Win2K?
Should i apply the service pack 2 for Win2K?
How can i solve this?
Thanks