I have found the smaller Brush DC Motors cause Electrical or RF Noise as their brushes wear.
If you have one of these DC Spindles with brushes and commutator dump them.
You can Identify a DC Motor or Spindle with brushes and commutator by looking at attached photos below.
They have replaceable carbon brush ports or they can be visible through vents on motor.
There are Cheep brushless DC Spindles out there, but if you are caving or cutting with small Vbits or tapered bits make sure it will do over 24,000 RPM or better.
Also make sure the Spindle is advertised as a Brushless DC or BLDC Spindle.
WHY?
As the brushes wear they cause arcing between commutator and brushes.
The arcing generates hi voltage and frequency spikes that are also present as RF.
This RF and EM will flow through you're CNC wiring and injected or block step signals.
THIS WILL GET YOU OUT OF TROUBLE!
I had one of these.
I added 2 Ferrite beads to the DC supply wires, the old DC motor has since been replaced with a brushless 800w water cooled spindles from Gpenny.
Pictured where the wires with the beads "Light Blue Rings"
Ferrite Beads are available in various shapes and sizes.
you can find them on old computer monitor, printer and power cables like this.