Thanks to all for their thoughts!
I use 3/8-12 Acme threaded rods and nuts for my drive systems and a high-temp lithium axle grease.
The CNC router is in the basement, so weather conditions tend to be standard. ; - )
Motor Tuning Acc & Vel are actually set below maximum already.
So the random Act of God may be the best explanation...
However, I did discover another artifact that may, or may not, have some bearing on the operation...
In the Config File, I typically have the program switch off (Enable command) my router and vac at the end of run or M30.
Because of other surprises in the past I also checked the Emergency E-stop as another Enable trigger.
During one run, the E-Stop was activated by a single line of code, yet the router was in a safe area!
And it would only happen at that line of code...
Nothing mysterious, just numbers for the X, Y and Z axis motors.
Turned off the E-Stop notification, and everything ran correctly.
Perhaps an exorcism is more appropriate...