I'm not technically savvy but for the past few months my 910xt has been having all the elevation issues like many are experiencing. My watch would start elevation at 20000m and then works it's way down to 0 then start again at 20000.
I had only been using it for running and bought a Garmin 800 for cycling as it pissed me off that much.
Anyway I'm back into full training and starting swimming again and using the watch in pool. All of a sudden the elevation is working again properly. I honestly believe it has something to do with it being fully underwater.
If you are having problems like me I suggest swimming with it and seeing if it fixes yours like it did mine.