ntpviz showing odd repeating offset for SHM(0) from gpsd

[shouldbe q931]

As a picture is allegedly worth a thousand words, I've included two pictures

daily view
https://pasteboard.co/J7p9VU5.png

weekly view
https://pasteboard.co/J7paicZ.png

As none of the other server offsets show this, I have presumed that
the repeating offset is not happening on the ntpsec side.

gpsd: 3.19-dev (revision 3.19-dev-2019-04-22T13:57:37.400598)
ntpd ntpsec-1.1.3+ 2019-04-22T13:02:45Z (git rev 5b9324c)

Running in a rPi raspbian stretch
4.19.66-v7+ #1253 SMP Thu Aug 15 11:49:46 BST 2019 armv7l GNU/Linux

gpsd is started with
/usr/local/sbin/gpsd -P /var/run/gpsd.pid -n  /dev/gpsd0

UBX-MON-VER:
 swVersion 1.00 (59842)
 hwVersion 00070000
 extension: PROTVER 14.00
 extension: GPS;SBAS;GLO;QZSS

Although the rPi also runs dump109-fa (an ADS-B receiver), I'm fairly
sure that this is not related.

Cheers

Arne