My machine could move to a location accurately but once it was cutting it’s movement was unpredictable. In searching for a fix I ran across this thread about trouble with the Hello World program. The problem they found was loose set screws on the motors. My Hello World was perfect but I thought I’d check the set screws anyway. Turns out that was my problem. Both y-axis motors had set screws so loose that the pulleys slid right off the motor shafts. I was able to remove the belts from the pulleys, tighten the set screws (making sure that I got them on the flat on the shaft) and reinstall the belts without throwing off my machine at all (my zeroes were still in the same place).
I thought I’d post this in a separate thread since most people won’t check a thread on Hello World problems if they’re already past that step.