@Eric and tbailey2,
Thanks both for your additional comments.
I can confirm that the HG612 Modem Stats Cooperation is DEFINITELY NOT SET and neither is HG612 Modem Stats running on any machine here. I have a mix of the following :
Mac-Mini running OSX (latest version)
Windows XP and 8.1
Raspberry Pi 3 x4 plus various other Model B - not all running at once. DslStats is running on one of the Pi 3s.
I have taken on board the suggestions about ensuring that the clock time on the Pi is correct, and will look into checking and updating this via ntp perhaps every 3 or 4 hours or more frequently if thought wise. Having said all this, it does strike me that the situation appears to have cured itself since late yesterday afternoon, and whether that is as a result of the various things I tried yesterday or that magic that technology provides, I am not really sure.
Anyway, for now, I'll keep an eye on things and let you know if the problem reappears.
Thanks,
toulouse