Airmar gps data

[lateral]

I Have a GH2183 & a G2183.
I am just about to install in my boat.
As a dummy run I have set up the two GPS’s/Hdg on the bench (one at a time) to compare output.
The G2183 (manufacture date 2010) exibits fairly stable position & COG data, whereas the GH2183 (2015)
has wildly swinging COG and a longitude 0.006 of a minute variation. (Latitude is only .002’) Roughly 25ft at 36°S.
Both were tested in the same environment with the same weather casting settings. (1Hz)
I understand at rest, the COG is going to swing with inaccuracies of position (which will stabilise with motion) but is
this normal for the GH2183? How can it be stabilised?
Any ideas appreciated.