ntpd dies nightly on a server with jails

Running a host with several jails on recent CURRENT (12.0-CURRENT #8 r315187:
Sun Mar 12 11:22:38 CET 2017 amd64) makes me trouble on a daily basis.

The box is an older two-socket Fujitsu server equipted with two four-core
Intel(R) Xeon(R) CPU L5420  _at_ 2.50GHz.

The box has several jails, each jail does NOT run service ntpd. Each jail has
its dedicated loopback, lo1 throughout lo5 (for the moment) with dedicated IP:
127.0.1.1 - 127.0.5.1 (if this matter, I believe not).

The host itself has two main NICs, broadcom based. bcm0 is dedicated to the
host, bcm1 is shared amongst the jails: each jail has an IP bound to bcm1 via
whihc the jails communicate with the network.

I try to capture log informations via syslog, but FreeBSD's ntpd seems to be
very, very sparse with such informations, coverging to null - I can't see
anything suiatble in the logs why NTPD dies almost every night leaving the
system with a wild reset of time. Sometimes it is a gain of 6 hours, sometimes
it is only half an hour. I leave the box at 16:00 local time usually and take
care again at ~ 7 o'clock in the morning local time.

When the clock is floating that wild, in all cases ntpd isn't running any more.
I try to restart with options -g and -G to adjust the time quickly at the
beginning, which works fine.

Apart from possible misconfigurations of the jails (I'm quite new to jails and
their pitfalls), I was wondering what causes ntpd to die. i can't determine
exactly the time of its death, so it might be related to diurnal/periodic
processes (I use only the most vanilla configurations on periodic, except for
checking ZFS's scrubbing enabled).

I'ven't had the chance to check whether the hardware is completely all right,
but from a superficial point of view there is no issue with high gain of the
internal clock or other hardware issues.

If there are known issues with jails (the problem occurs since I use those),
advice is appreciated.

Thanks in advance,

O. Hartmann