Course malfunctioning when running all the way through

I built 5 courses, each is about 120-200 slides, most slides are software simulation, a combination of demonstration and training with a few intro slides (title, learning objectives - captions only) and a quiz.  There is no branching, no video, no audio.  4 courses were done in Captivate 2017, the last one was done in 2019, but the issues are very similar.

3 courses seem to be working just fine, but the last 2 tend to throw up errors, usually in the second half of the project.  The most common errors are:

a) background doesn't load - usually only captions and highlights show up, no background

b) "next" button doesn't work - seems frozen (don't even change on rollover)

None of the problematic slides have any action - not on enter, not on exit, not on the slide.  None of the slides are particularly complex, most have a single click box and a caption, so it's not particularly likely that I'm failing to diagnose a major design flaw.  For most of those slides, the only thing I've really done to the automatically-generated simulations is edited the captions.

Furthermore, if I re-start the project "from this slide", and set it 1-2 slides prior to the problematic slide, 99% of the time, the slides work just fine.  The problem seems to only arise when they are run in a long sequence.  In other words, slide #63 doesn't work, but same slide when it's run from the middle of the project, works just fine as #2-3 or 10.  So, it almost feels like the project is too long, rather than something is wrong with individual slides?  Should I consider breaking my projects up in smaller chunks?  If so, what is a good number to target?  Nearly all the malfunctions happen at ##60-70-80, so is 50 slides a better, safer project size?

One slide that I was able to zero in on literally had just background and 1 highlight, the whole slide is 1,5 seconds, no video, no audio - highlight would show up, but background wouldn't.  I really have no ideas for what I can trouble shoot on it (again, no actions).  My only guess is that this slide was a duplicate of another slide.  Could that somehow corrupt the background?

Another hypothesis I'm working with is copy-pasting stuff.  I've been copying the shells and adding new content to them.  I usually delete all but 3-4 slides that are the same in every course.  But could that still result in some run-through issues?  If so, would copying-pasting slides in a new shell resolve it, or would it copy-paste the issues as well?

I'd appreciate some insight.