Hi,
Does mach3 or perhaps the control board have a builtin feature where the limit will trip if the tool encounters too much resistance?
No, Mach3 does not do that.
My guess is that electrical noise is getting into your limit switch wiring.
In the first instance you need to fit line filters and/or line reactors to tame the noise from the spindle.
Thereafter look to shielding your signal wiring, load the signal lines with an impedance chosen to damp noise optimally.
Most beginners spend all sorts of trouble shielding and filtering signal lines without ever considering reducing the noise output
of by far and away the noisiest (electrically) component....the spindle.
Craig