We had a sudden 15° shift occur. It was almost the same as our variation here so thought that was it. Recalibrated the compass but the performance was still a bit sloppy. A while later I was moving some stuff around in a locker and picked up the spare freshwater pump to get at some stuff when the light bulb came on. It was right above the fluxgate below the sole. Moved it away and recalibrated again and was back to normal. Big magnets in that pump. Yet another self-inflicted wound! Could yours be something similar?