The shimmering I described was called "double traces" and a fix was described here:
https://groups.io/g/TekScopes/topic/77914568#175831
by Jack2015, where adjusting the value of R318 eliminates it.

I've tried it, and it does make a noticable improvement, but for me not as good as he had
and showed on youtube. That's just a comparator threshold, so I don't understand how
changing the step threshold could help.