Hood
May be all wet here, maybe someone can chime in, but here's goes.
You have the following:
1-supplied voltage- say 60 hz
2-spindle sensor frequency to mach - 1 pulse per rev
3-VFD generated dominant freg- i can measure but most would not be able to do
4-the stepper pulses per unit-

Now you want to be away as far as you can go from any fundamenetal or harmonic....thus in terms
of suppled voltage 60,120,180, ...etc you want the spindle sensor being at 30, 90, etc.
So your input / output MACH pulses have less chance of having an intereference. Now if it's brute noise,
that's a diifferent story,. If that's the case then it would not be so intermitant and the grounding, shieldnig etc will have more effect.
A little far fetched but if the field builds up i can see where you can get some "goofy" pulse or interference
that could make the X axis not behave yet contnue. But like i said, a little farfetched.
Just some out of the box thinking,
RICH