Estop Triggering; why?

I've reconstructed a system using Gecko Drive G320X Servo Drivers and have got it to work but I am going crazy with seemingly random Estop interruptions in Mach 3. I have homing and limit switches with shielded cable and am NOT getting errors from them. I was able to run 1000 lines of G-code without interruption twice if I turned off the 1500w VFD and spindle. This suggests the spindle and VFD are responsible for the problem. Conversely when I do an actual cutting job I will get an Estop error at about 400 one time and maybe 800 lines the next time.
Why is the Estop triggered? Can anyone suggest a technique for tracking down the problem. I have got an oscilloscope.
Suggestions much appreciated.