Problems with Altimeter

Thanks. 

My run this morning was between 22 and 27C.

Seems odd that sitting at home would yield 38C.

henzim said:

Seems odd that sitting at home would yield 38C

Yes, that seems odd to me too.  Normal body temperature is 98.6F and your watch is displaying nearly 101F while sitting.  I'm at a loss.  I can't imagine the Instinct Solar operates at a higher internal temperature than my 945, but I don't know for sure.  Anyway, I hope you get this sorted out, as I don't think I've been much help and probably threw out to many "what if's" and made solving your issue more difficult.  

I was charging my watch now. The temperature has dropped from 31 ° C to 23 ° C, the room temperature is about 21-22 ° C. So when I realize that the watch may have warmed up slightly by charging, it measures really accurately. The temperature of the hand at rest without activity is the difference of about 8 ° C.

Finally, thanks to the amazing customer support of Amazon, I now have a new working(!) watch. Even though all return deadlines had long past and my watch was regularly used and by no means in mint condition, Amazon stepped in and organized a replacement for me.  

A big *thanks for nothing" to Garmin! 
I am as disappointed with Garmin as I am happy with Amazon.
Since day one Garmin did not care about my concern and tried to downplay the problem. In a reply to my OP, where I described my disappointment with 60 m altitude drops during 45 minute runs, Garmin - Chris just said my values were excellent and I could always adjust my altitude in Garmin Connect afterwards (yeah, very useful for an outdoor watch.) 

Garmin - Chris said:

What you are experiencing is considered excellent for a consumer grade barometric altimeter

My local Garmin support basically told me the same, "GPS-only would be worse" and "I should calibrate more".

I want to thank all users who joined the discussion and helped me keep faith that the readings I were seeing were worse than could be expected. Special thanks to Former Member, trux, TeaRacer.

I have had my my new watch for about two weeks and now I am finally  seeing consistent, good altitude values. Whether I walk into town and back or go for a run or bike-trip I end up at approximately the same altitude I started on. Sometimes I loose/gain 5 meter per hours of activity, once I lost 7 meter (see below), but it is typically only 1-2 meters.

I had both watches (old and new) for a couple of days. Here is the result of my first run with both (same calibration settings, same GPS settings, both calibrated before runs, etc. etc...) 

Old Garmin Instinct Solar	New Garmin Instinct Solar

I am also very happy I swayed to a different brand when I bought a sports watch for my wife recently.
This (cheaper) watch also showed me, that altimeters can work.
Although I am currently happy with my Garmin watch, I will never buy one again nor recommend the brand to anyone.
If you are unlucky enough to get a faulty hardware or experience problems, you are out of luck and it will likely leave you unhappy and frustrated. (Unless perhaps you were lucky enough to have bought it through amazon)   

So I'm glad it's better!