Garmin 955 Altitude Problems

Hi, unfortunately no, except for hiking one day...it was relatively accurate at 960m and after that at about 1075m.  I have been ill twice before that + bad weather and just recovering from covid so I wasn't on my regular running route or anything except that one i mentioned and it was OK. there was some discrepancy between freind's Coros Pace 2 I think and FR955 in total ascent. At home it constantly shows accurate altitude for a while now...but I wasn't sweating (sensor hole).

I have the same problem with altitude accuracy...last time i was skitouring was beautiful, stable and sunny weather 2 days in row.
The altitude on the peak table was 1946 masl. and 955 was 1933 baro and similar on gps...i was there 15 min. and nothing changed. Im using all gps + multi and 1sec. During the trip were altitudes /gps+baro/ many times the same, but on peak were both wrong. And its peak with open sky. The same problem i noticed on trips before, peak 1477 masl.  and my 955 was 1470. My 8 years old Suunto Ambit2 is only 1-2 meters away !!! its software or hw problem ? look at the attached images.

Well, that is better than what I was getting at about 430m, almost 20m less than that...sometimes. Have you calibrated before? Have you calibrated the watch on the first peak? I was asked to try to soak the watch for 15-20 minutes in lukewarm water. I guess it helped that time. There are videos on YT...i.e. 2 FR955 watches, one day one is much lower than real altitude on some peak, the other day it is vice versa.

I have calibrated the watch sometimes...but, the calibration is still based on gps signal. When you start your activity, the watch is searching for gps and  few minutes  after gps fix, the barometric altitude is "the same" as gps altitude - called Fused Alti on my old Suunto Ambit2. But, i dont kow if it works, bcs its it not possible that the gps altitude is wrong too...and the fix with water ? :D should i go swim before any activity fot 20min ?

I know, I had Suunto 9 Baro. It should work in coordination with GPS, but on my Garmin it doesn't...as in your case. The position is right, the altitude is known and yet it misses so much...I guess because it relies only on barometer. My altimeter is on Sensor mode - Auto and Auto Cal. - ON. You should have had calibrated at the table on first peak and then compare on the other...to see if it is correct. I checked it on my last hike that I mentioned here and it was pretty close, but just cannot go for a run on my usual route till this year to see how it behaves now at 432m. Maybe next week if I go for another hike this weekend. Fix with soaking was suggested by Garmin support. I thought something like that...I don't want to soak it in water every time before the activity. I don't like the placement of the hole also, under the watch, some others I had (and F6, F7 it think) have barometer sensor hole on the side. I don't know if it matters, but the sweat and dirt can easier block it on the bottom...in my opinion.

Have you tried turning off the barometer mode and just alti only? I found that my altitude got more accurate after that. I was losing almost 100m per 1000m climb previously. After turning off baro, it was a few m difference.