FreeBSD 10.2-RC3
   ntpd 4.3.68

Hal, thanks for mentioning this on the mailing list. I should have spoken up sooner. I've been seeing this for a LONG time (2-3 years?) but I workaround by replacing hostnames with IP addresses in the config file 'server' statements and then forget. Every several months, I look at the config, scratch my head, put the domain names back in, and then remember!

I have been seeing this ONLY on an Intel Xeon E5-2603 (the biggest of our machines). It has two CPU's each with 4 cores and, to me, this smells like a thread problem. This server is now running FreeBSD 10.2-RC3 but I have seen this same problem on this server on earlier versions as well (definitely FreeBSD 10.1 and 9, not sure about 8).

Just now I edited the config file to use domain names for server config and produced this dump. Like Hal, this doesn't happen EVERY time ntpd starts but, for me, it is the rule rather than the exception.

rwsrv08# gdb /usr/sbin/ntpd /ntpd.core
GNU gdb 6.1.1 [FreeBSD]
Copyright 2004 Free Software Foundation, Inc.
...
Core was generated by `ntpd'.
...
#0  0x0000000800678fe2 in _rtld_atfork_post () from /libexec/ld-elf.so.1
[New Thread 801c07400 (LWP 101340/<unknown>)]
[New Thread 801c06400 (LWP 100229/<unknown>)]
(gdb) bt
#0  0x0000000800678fe2 in _rtld_atfork_post () from /libexec/ld-elf.so.1
#1  0x0000000800679349 in _rtld_atfork_post () from /libexec/ld-elf.so.1
#2  0x00000008006749f4 in _rtld_is_dlopened () from /libexec/ld-elf.so.1
#3  0x0000000800673e3a in _rtld_is_dlopened () from /libexec/ld-elf.so.1
#4  0x0000000800670ea0 in dlopen () from /libexec/ld-elf.so.1
#5  0x00000008013e9025 in _nsdbtaddsrc () from /lib/libc.so.7
#6  0x00000008013e37e4 in _nsyyparse () from /lib/libc.so.7
#7  0x00000008013e96a1 in nsdispatch () from /lib/libc.so.7
#8  0x00000008013cd011 in getservbyname () from /lib/libc.so.7
#9  0x00000008013ccf19 in getservbyname () from /lib/libc.so.7
#10 0x00000008013c9a33 in getaddrinfo () from /lib/libc.so.7
#11 0x00000008013c7358 in getaddrinfo () from /lib/libc.so.7
#12 0x00000000004382c6 in blocking_getaddrinfo ()
#13 0x0000000000439190 in blocking_child_common ()
#14 0x000000000043b3b9 in blocking_thread ()
#15 0x00000008010bd7d5 in pthread_create () from /lib/libthr.so.3
#16 0x0000000000000000 in ?? ()
(gdb) q

When this happens, syslog shows...

Aug 18 09:30:24 rwsrv08 ntpd[17587]: ntpd 4.3.68@1.2483-o Fri Aug  7 02:03:11 UTC 2015 (1): Starting
Aug 18 09:30:24 rwsrv08 ntpd[17587]: Command line: /usr/sbin/ntpd -g -w 120 -N -c /data/ntpd/ntp.conf -p /var/run/ntpd.pid
Aug 18 09:30:25 rwsrv08 ntpd[17588]: proto: precision = 1.118 usec (-20)
Aug 18 09:30:25 rwsrv08 ntpd[17588]: Listen and drop on 0 v6wildcard [::]:123
Aug 18 09:30:25 rwsrv08 ntpd[17588]: Listen and drop on 1 v4wildcard 0.0.0.0:123
Aug 18 09:30:25 rwsrv08 ntpd[17588]: Listen normally on 2 GFNX 203.58.93.40:123
Aug 18 09:30:25 rwsrv08 ntpd[17588]: Listen normally on 3 GFNX [2001:8000:1000:1801::5001]:123
Aug 18 09:30:25 rwsrv08 ntpd[17588]: Listen normally on 4 lo0 [::1]:123
Aug 18 09:30:25 rwsrv08 ntpd[17588]: Listen normally on 5 lo0 127.0.0.1:123
Aug 18 09:30:25 rwsrv08 ntpd[17588]: Listening on routing socket on fd #26 for interface updates
Aug 18 09:30:25 rwsrv08 ntpd[17588]: ff08::101 8811 81 mobilize assoc 36615
Aug 18 09:30:25 rwsrv08 kernel: pid 17588 (ntpd), uid 0: exited on signal 11 (core dumped)

Please let me know if I can be of any assistance.