Dan. These lot will correct me when im inevitably wrong, but ensure you velocity and acceleration values are the same for both X and Y axis

make sure your acceleration is not too slow

that always sort it for me. Im not too sure of the technicalities of why. There is a code you can inject... like 'G60' or something you append to your gcode file before opening with mach. but it makes the machine stop at every change of direction. but I managed to sort it with the explanation as above.

Matt