Didn't appear to be multi-start. I quickly checked pitch with a caliper, mainly to make sure that it was 3mm and not 8tpi, bit it's not the most accurate measurement to do. The x54 move giving about 56mm movement is slightly odd as that doesn't tie up with either pitch. Unless steps per has got itself changed. The current motor mounts are a bit crude (plan is to remake) but I can't see how any flexibility in that or missed steps would give too much movement. X and y axes seem to move smoothly enough. Should have done a bit more testing...