Howdy Cxixer,
As you are using steppers on your machine, perhaps you are seeing the effects of resonance in them.
This has some very undesirable effects, including unexpected stalling.
One way to look for this is to jog the affected axis back and forth, all the way between limits, at varying speeds.
Under certain conditions, for a given speed, the motor may begin to vibrate. The effect builds up over a short period of time, and the motor suddenly stops.
This caused havock with my machine, until I discovered the curative powers of 'resonance dampers'
.
Here's hoping this helps.
Best wishes,
Steve.