squid, DNS and no buffer space

On a busy squid proxy server, users are seeing performance problems and 
squid is logging the following:

2024/12/02 15:00:06 kid1| comm_udp_sendto FD 9, (family=2) 127.0.0.1:53: (55) No buffer space available
2024/12/02 15:00:06 kid1| idnsSendQuery FD 9: sendto: (55) No buffer space available

As the first line suggests, named is running on the host itself (and is 
set to forward to another host on the same LAN). Both should be caching.

# netstat -m
31927 mbufs in use:
         19124 mbufs allocated to data
         12800 mbufs allocated to packet headers
         3 mbufs allocated to socket names and addresses
32 calls to protocol drain routines

#  sysctl net.inet.udp
net.inet.udp.checksum = 1
net.inet.udp.sendspace = 18000
net.inet.udp.recvspace = 41600
net.inet.udp.do_loopback_cksum = 0

Any ideas of things to tweak or investigate?

-- 
Stephen