All axis motors make random steps when the spindle is on.

I have a 2020 3-axis mill with LPT interface.
When I turn the spindle on, it runs fine for a couple minutes, but then all three axis motors begin to make random jumps, mostly in the same "positive" direction (Z-axis tends to go up, and Y axis towards the back, for example).

Is there anything I can check in my software settings? It's a Debian dedicated installation with an i7 machine with PCIe LPT board.

Sounds like electrical noise from the spindle drive getting into the step/dir signals
so probably needs a hardware fix rather than software.

Does the spindle have a VFD? these tend to be bad noise radiators

How are your step drives connected to the LPT port?

Is there any power/spindle wiring next the the step/dir wires?

The control board with the drivers are in an enclosure like this: ae01.alicdn.com/kf/HTB1J41SLXXXXXXbXXXXq...ont-b-CNC-b-font.jpg

The spindle is wired to the spindle motor port on the back of the box; there are not inputs controlling either speed or direction, just on/off.

The board in the docs is pictured like this: drive.google.com/file/d/0B-TeFG5-eqWZT3l...SlE/view?usp=sharing -- I have not opened the enclosure to look at it yet. The spindle power connector is right next to the three motor connectors.

Personally I would ground the main frame if it is not already done

Thanks, that's the first thing I will do now.

Is there a way to tell if the interference is on the signal side (digital inputs get flipped when the computer does not send anything) or on the power side (motor gets triggered when the driver did not send anything)? I don't have an oscilloscope at hand to see what the motors are getting.