This is the first layer, taken when it has only printed black. It's being printed on an adhesion sheet which was used for another build previously, hence looking like there's something under the layer.
Thank you! I've been working on it on and off for an embarrassingly long time, it's great to see it finally coming together.
I suspected that as well, so I printed the same gcode file a second time and the errors showed up in the same spots on the first layer. I guess it's possible something about the printer is causing the interference which could make it deterministic. But I will make sure there are no other electronics are near it and take the other steps you suggested with the cables.
I've included the gcode for just the black part of the first layer that I printed. Looking at it in NC Viewer it looks like it's not a gcode issue. However I see that I had arcwelder enabled; I should have turned that off before submitting a bug. I'll try slicing with that disabled and see if it makes a difference, but given those lines already don't show up in the gcode it seems unlikely to make a difference.
It may be a few days until I can make the suggested changes and run some tests, but I'll let you know what comes of it. Thank you very much for all the help!
layer1.gcode