I had a ongoing problem with the Z axis not stopping sometimes, not sure if this is relevant but i checked everything and could not find a problem. However one day when using my CNC machine my screen showed that zone alarm was asking for permission to allow my computer to accept a program at the same time the z axis was not stopping, luckily it was a z + command so it just hit the overtravel switch. I had a thought that my broadband connection was sending signals down the printer line which was now my Mach3 connection, I was not using the internet at the time but the computer was linked via my wireless connection. Since then I disconnect broadband whenever i use my CNC machine. This was 4 months ago and the z axis has behaved it self since. Just a thought, maybe totally wrong but worth thinking about. Also some of the security software does background checks of you software, this could also send signals to Mach3 via your connection to your machine. Jim