Yes, tried multiple stl files, including the ubiquitous Benchy straight from the headwaters.
Thanks all for the advice and head scratching. Things seem to be working again. Thanks also for the good link with gcode.ws I hadn't seen that before.
Honestly I'm not 100% sure what caused the problem. BEST GUESS here is I had a screwey SD card, which would make sense. Good printer? Check. Good GCode? Check. Troubleshooting says the interface between the two is suspect, and I should have gone there immediately. So if anybody else sees this issue, reformat your card and or get o