Erratic SOG from Garmin GPSmap78

[Draughty]

Hi all,
I have for years been using SOB nav software on a laptop with the GPS data being fed via serial-USB adaptor from an old Koden KGP-912 GPS navigator.
Part of the Koden screen sort of misted over and it's been through 2 date changeovers (can't recall the proper terminology here) so any date/time data attached to track points for example are nonsense. I hooked up a Garmin GPSmap 78 I had as a spare, using the same serial-USB adaptor and attaching the old external Koden antenna. Position data is fine but SOG and direction are erratic. If for example I'm doing a steady 6.5 to 7 knots STW the SOG displayed on the computer screen varies every couple of seconds or so, anywhere between 4.5 to 9 knots. The course arrow emanating from ownship swings about 45 degrees either side of ahead.
Neither the Garmin nor the SOB manual seem to address this.
Any ideas out there?
Rgds

[Draughty]

Tried to edit above but just get little window asking for reason for editing?
Thought after posting above to try disconecting the external antenna and that seems to have worked, on anchor anyway. On anchor SOG was flicking between 0.0 and 0.5 knots in 0.1 increments, now a steady 0.0kn.