Paul, thanks for the interest.
Based on your thoughts, I just zeroed the router and moved it all over the table using different step sizes and speeds. I jogged around for about half an hour, and it always moved back to my set zero. I was hoping to see something weird, but I didn’t see any free form moves.
You’re right, the random moves seem like spurious signals. My LongMill has been operating since December, with no changes - same cables, router, location, PC, etc., there are no motors running nearby, and it has been trouble-free. I’ve used it for hours at a time with wood, brass, steel, and SS. I did switch to the magnetic dust shoe a couple weeks ago and thought I may have damaged a wire when I was swapping parts, so I checked them thoroughly with a meter while flexing them and found nothing. A while back I moved the motors wires off the controller and onto a Ramps board to run the 4th axis, but they’ve been back on the controller for a while and I’ve run a probably 100+ hrs since with no problems.
I thought maybe file was damaged but I compared it with the original using WinMerge and it’s identical.
Oh well, I’ll keep probing. Thanks again for the help.
Bill