Strange DNS problem

[Bakul Shah]

I am running a local dns server (bind9).  It works ok for the
most part but a number of domains do not resolve and I have
not been able to detect any pattern. For instance

$ host weather.com 
Host weather.com not found: 3(NXDOMAIN)

Here is the tcpdump output (on the internal side):

21:45:14.662220 IP 192.168.125.253.20204 > 192.168.125.9.53: 16136+ A? weather.com. (29)
21:45:14.662696 IP 192.168.125.9.53 > 192.168.125.253.20204: 16136 ServFail 0/0/0 (29)
21:45:14.663090 IP 192.168.125.253.59705 > 192.168.125.9.53: 4383+ A? weather.com.bitblocks.com. (43)
21:45:14.663193 IP 192.168.125.9.53 > 192.168.125.253.59705: 4383 NXDomain* 0/1/0 (94)

What is strange is *nothing* goes out of my network when such
a failure occurs!  [My gateway machine does NAT on the
internal side but the symptom is the same on the gateway
machine]

I ignored the problem for a while but recently I noticed a few
more domains do not resolve! May be nothing has changed since
I didn't try these domains before.

This is on 8.2-release (but the symptom was the same with 8.1
and may be earlier).

I checked named.conf and AFAIK it seems ok -- it has not
changed substantially since 2006 (I moved to a different IP
block and had to change my ISP's dns server address -- it is
my secondary).

I'd appreciate any hints on how to diagnose this.  Thansks!