Birchy,
Sorry can't help as never had the problem.
Don't run Linux in case that's the problem. having said that fitted rack loads of the 542's and similar motors.
Are these zapp motors ? Reason i ask is I had some [ not sure where they came from ] where the wiring colours wasn't what it seemed. Having said this bad wiring will be with you across the range, not just in a band.

So far all my conversions have been done with straight 4 core wiring to the steppers, never had an interferance problem to warrant screened cable, yes I know it makes sense but just never had a problem.

Done about five WM16's to date plus a few WM14's