Larry (and rb) -- thanks so much for this. Never thought in terms of a heading "sensor" -- didn't know there was such a thing. Have just assumed it was some sort of calculation from GPS data.
Would this have any relationship to Hagar's late comment about stowed stuff affecting the fluxgate? A fluxgate/sensor relationship? I don't believe the fluxgate was an issue, but we'll check that.
I will also take a look at the CP setup for COG next time I'm on the boat. If I can find that and there is a choice between using the sensor or the GPS for COG, what would you normally recommend? And if there is a choice, would my enroute fixit have been to change from one to the other?