Noticed something about this issue today - I can do the movement w/o shaking, or I can lift the shoulders a bit further, or push the lower back into the ground a bit more firmly, and elicit the shaking response. Also, on occasion the shaking will start mid-set then subside with no changes in position (which is a bit weird).
Given the above - I am going to experiment with staying within the ROM that doesn't produce any shaking and see how it progresses. If I get to 60s holds and the ROM still hasn't been able to get increased then I will revisit.
- Chris