There are lots of varibles here. My geuss is you have alignment issues.
I also had racking issues and found that my linear guides were not aligned accurately enough. After a lot of measuring shimming check more measuring and shimming and slective loosinging and tightening of bolts then more shimming it is working great.
My machine has lower powered steppers than yours (540 ozin I think) and is also direct driving my ball screws which have a 16mm pitch. I intentionally choose a coarse pitch to keep the motors spinning slow for more torque. Machine travel is around 40" x 60" so not smalll but not teribly large either. Larger machines are harder to get everything in alignment.
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