Mine is acting strange as well. Tried a factory reset and it was ok for a day, now it´s starting to jump in the baro again. I think it´s a software issue but will probably send it back unless garmin goes public on this, aparently, big problem with the fenix.
Are you facing stormy or fast changing weather right now?
Because this can cause problems with the barometric sensor.
Let's be realistic here - weather-driven changes (excluding an hurricane) in ambient air pressure are typically the equivalent of around 200m/600ft in altitude. The changes being seen here and many others on these boards are far in excess of that.
The changes in altitude are more likely to be caused by static electricity (from colder, drier weather and the technical fabrics in some long-sleeve athletic tops) affecting the barometric sensor. Soaking it in soapy water seems to resolve the issue at least temporarily.
