No sure the best place to post this, but I have a custom made gantry mill running off Mach 3. When I set the on screen MPG to step mode, the mill sometimes just stops moving.

It appears to work perfectly fine in Continious mode.

Once it stops responding to Step moves, I have to hit RESET to get it back working again.

Where do I start trying to find out why this is happening?

It's a custom built machine which I bought from someone already built. It works fine but this issue makes setting up a pain.

Is this likely to be a Mach 3 issue or a controller issue or a noise/emc issue?

The controller is a UC100

Any tips appreciated!