I am able to execute fast tremolo picking (without string hopping), with both an elbow approach and a usx (lightly supinated). The latter especially feels nice and relaxed and fluid. However, I am unable to slow this down. Anything in the range of 80-140 and my hand bounces up and down like crazy. My old string hopping habit comes right back. I’ve tried everything I can think of, over many months, to tamp this down but I’m at a loss. It’s a horrible feeling. What are some of the most robust ways to trick myself out of this habit? I’ll try anything at this point…
I had this problem as well, since I spent way too many hours trying to practice the wrong technique. I think what finally helped me was to do a rest stroke on the non-escaped pick strokes. That way you get a tactile feedback from the next string and force yourself to remove the hopping movement.
This already feels really promising! One other challenge I have is the muscle on the underside of my forearm often tenses up. I’m trying really hard to relax that, but it’s a challenge. When I’m able to relax it, my playing is more fluid. I think that tensing of the underside of the forearm is a relic of string hopping. I had to flex there to bring the pick back down.
I’d highly recommend the Technique Critique feature of the platform. If you’re logged in on the main site and go to this url it should walk you through the process. It’s a huge benefit of having a membership - virtual guitar lessons with Troy and Tommo??? No brainer, and you’ve already paid for it 
https://troygrady.com/dashboard/tc/
There was a recent video Troy made (can’t find it at the moment) where he showed a player who was learning a new motion and at some other speed, the motion reverted to “the other one” that the player already knew. The idea was something along the lines of the neuro system just executing “programs” that it knows. If you’ve been a string hopper for a substantial time period, and you play at or just slightly above string hopping tempo (80 - 140 like you mentioned) it makes sense that you’d revert to the old way.
I’d actually noticed something similar happening to myself even when practicing a new motion at higher speeds (170 etc). My tremolo was perfect, but the minute I introduced fretting, the motion changed. It didn’t turn into string hopping, but it went from the forearm/wrist blend USX motion I was working on, to my wrist DSX motion that I was already comfortable with. It was what I knew!!! Un related, I’d discovered my hand sync was poor on certain patterns, so I wrote a drill to address both problems at one:
It should encourage the tremolo motion throughout. Feel free to substitute a different fretting hand “shape”. I chose the one I’m best at because once you get good at one thing, it’s easy to branch out from there.