Searching this group I found someone who had the exact same problem as I am
having. Unfortunately the thread fizzled out (back in November last year)
and no fix was posted.

Customer has SBS6 with ISDN dial-up to ISP, and 4 Windows 2000 workstations.
I've been having ongoing problems with NAMED from the start. As a workaround
the workstations' DNS settings were pointed to the ISP's DNS servers, rather
than (and NAMED). Problem is of course, potential for lots of
unneccesary dial-up.

All versions of NAMED I've tried have this problem, even the one from
dns512f.exe. The NAMED forwarders are in place, and I've disabled the DNS
client service on the W2K workstations. If the line drops and connects
(dynamic address), browsing at the workstation fails. Reloading NAMED once
the connection is up allows browsing to work. A binding issue?

I've had a look at the NAMED screen with the verbose switch added. It just
looks like the ISP's and root servers don't have the request in their
databases. Please find attached a section of the DBGLOG.

Not currently using BM, but might have to go down that route. Need to throw
some more memory at the server first. As is the case with smaller customers,
reluctant to spend cash.

TIA, Mike.