FIXED!
Thank you everyone for your recommendations.
A special thank you to "Cincytrains" for being the first to remind me of the basics -- I cleaned about 20-25 feet of track spanning the area of three of the four locations where LRS was dropping out.
Both of these new locomotives ran flawlessly repeatedly and consistently, not just over those three areas, but the entire loop (approx. 72 ft), at the "restricted speed" setting -- both forward and reverse.
Thanks again.