Kind of difficult to say… It’s a small one which a friend saved from the trash and it looks very custom built. I wrote about it here, there are also a few pictures. It uses an Arduino Uno with cnc-shield and GRBL.
The problem appears to be only in one direction (X) so far, so maybe that’s a pointer?
It could be that I use a ribbon cable for all the motor stuff, so no shielding (parasitic capacitive coupling), or maybe too high feedrate (300 mm/minute, at a depth of 0.02mm with a 30° V-bit). Or maybe there is some dirt on the rail? or I did not set the current for the motor correctly? or something else?