[CentOS] Networking issue (after latest 5.1 update?)

I'm not *entirely* sure but this began happening at about the time I
installed the latest kernel updates for CentOS 5.1
(kernel-2.6.18-53.1.6.el5.i686.rpm from January 24).  Shortly before
that I changed the encryption on my wireless router and switched from
ndiswrapper back to the standard bcm43xx driver, but I'm pretty sure
the network was OK before that update for the simple reason that I was
able to download the update itself.

In any case, for the past week or so wireless network connectivity
from my CentOS 5 laptop has been terrible.  The wireless router is set
up as my DNS server, and connectivity is as expected for
wired-connected computers and wirelessly from my Mac, from the game
consoles, from my work laptop (currently running ubuntu), and from the
CentOS laptop when re-booted into Windows XP.  So the problem is
restricted to CentOS 5.

It takes minutes to do DNS lookups, and to establish a connection in
the first place takes so long that "stateless" stuff like web surfing
ranges from impossible (timeout) to merely unbearable.  The automatic
software updater hasn't succeeded in connecting ("Details: None")
since the 24th.  However, SSH connections seem to work OK (if perhaps
a little bursty) once I manage to get connected (often by ssh'ng to
the destination by IP address rather than by name).

I can ping computers on the wired LAN on the other side of the router:

PING 192.168.0.100 (192.168.0.100) 56(84) bytes of data.
64 bytes from 192.168.0.100: icmp_seq=1 ttl=64 time=0.094 ms
64 bytes from 192.168.0.100: icmp_seq=2 ttl=64 time=0.084 ms
64 bytes from 192.168.0.100: icmp_seq=3 ttl=64 time=0.082 ms
64 bytes from 192.168.0.100: icmp_seq=4 ttl=64 time=0.084 ms
64 bytes from 192.168.0.100: icmp_seq=5 ttl=64 time=0.085 ms
64 bytes from 192.168.0.100: icmp_seq=6 ttl=64 time=0.088 ms
64 bytes from 192.168.0.100: icmp_seq=7 ttl=64 time=0.085 ms
64 bytes from 192.168.0.100: icmp_seq=8 ttl=64 time=0.085 ms
64 bytes from 192.168.0.100: icmp_seq=9 ttl=64 time=0.086 ms
64 bytes from 192.168.0.100: icmp_seq=10 ttl=64 time=0.087 ms
64 bytes from 192.168.0.100: icmp_seq=11 ttl=64 time=0.087 ms
64 bytes from 192.168.0.100: icmp_seq=12 ttl=64 time=0.085 ms

--- 192.168.0.100 ping statistics ---
12 packets transmitted, 12 received, 0% packet loss, time 10997ms
rtt min/avg/max/mdev = 0.082/0.086/0.094/0.003 ms

But when I ping the router itself:

PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
64 bytes from 192.168.0.1: icmp_seq=3 ttl=64 time=32.1 ms
64 bytes from 192.168.0.1: icmp_seq=4 ttl=64 time=2.36 ms
64 bytes from 192.168.0.1: icmp_seq=5 ttl=64 time=1.89 ms
64 bytes from 192.168.0.1: icmp_seq=7 ttl=64 time=1.76 ms
64 bytes from 192.168.0.1: icmp_seq=8 ttl=64 time=1.80 ms
64 bytes from 192.168.0.1: icmp_seq=9 ttl=64 time=1.78 ms
64 bytes from 192.168.0.1: icmp_seq=10 ttl=64 time=1.84 ms
64 bytes from 192.168.0.1: icmp_seq=12 ttl=64 time=1.94 ms
64 bytes from 192.168.0.1: icmp_seq=12 ttl=64 time=3.58 ms (DUP!)
64 bytes from 192.168.0.1: icmp_seq=13 ttl=64 time=1.88 ms
64 bytes from 192.168.0.1: icmp_seq=14 ttl=64 time=1.99 ms
64 bytes from 192.168.0.1: icmp_seq=16 ttl=64 time=1.76 ms

--- 192.168.0.1 ping statistics ---
17 packets transmitted, 11 received, +1 duplicates, 35% packet loss,
time 15997ms
rtt min/avg/max/mdev = 1.765/4.562/32.120/8.323 ms

At one point I had 15 consecutive DUP packets during  a ping run
before things finally settled down.

Any idea what could cause this, or how to diagnose it further?  I've
run "arping -D" and there is only one responder on 192.168.0.1.