Just did Tour of Fire and Ice 100% Kickr difficulty. Half way in my chain skips a tooth here and there. I just got done aligning it so I am trying to figure out if I need a new drive train/derailleur. 2/3’s up the alp my chain is skipping every pedal now. I am unable to use my power kicks to get up the alp 
So in my mind I definitely need a new derailleur. 5/6 the way up and my gears are shifting randomly? Im thinking how is that even possible. When I aligned my derailleur it had a little flex but now it seems to be going wherever it wants even when i practice good riding form. So I make it up the alp and hop off to take a look at the damage. I’m thinking maybe i bent some teeth or ripped em off. In the past I folded a sprocket so it wouldn’t surprise me. I go pulling on the derailleur and my bike moves. IT WASNT EVEN IN THE AXLE! Lol come on. it was sitting on top of the axle lol. No wonder I was having alignment issues. Now I got to realign everything. O well live and learn. I’m lucky I didnt do any damage or hurt myself, well I mean in a non workout kinda way. My legs are beat.
I’m not a mechanic, but in the past when I have had chain skipping it’s because of a worn cassette. I hope it’s your problem as this is an easy fix. Just a suggestion.
Good luck!
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rode last night 40 miles 2200ft ele with no issues. somehow or another my bicycle frame was sitting above the axle. It worked itself free from a proper mounting.