Thanks for the comments, guys.

Don't think that it's pull-up problem as the IP/M has a bit more sophisticated input arrangement that does not need these. In any case, the indicator led on the switch itself flickers which suggests a problem upstream of the IP/M.

Grounding - I'll come back to that.

Don't remember anything in the docs that says that IP/M does not support combined home/limit switching; that bit seems to work and it's just the unreliable home switch trip (again, you can see this at the led without considering the motion controller) that does not work. In other words, the IP/M seems to work as per standard Mach3 functionality.

Now - grounding. For testing the control box is sat on the floor in front of the machine. So the wire from the star point in the box is not connected to the mounting bolt as intended. I shall sort this out, but I seem to remember noticing that there was not a clear switching point when testing using a bench power supply, so I'm starting to suspect that this is not noise-related.

Around 4 am I woke up thinking about hysteresis. When I was reading up on these switches, I remembered seeing something about this so I have just gone back to check. Typically, these things should have a hysteresis range of 10-20% of the switching distance, so about 0.4-0.8mm in my case. Switch-on and switch-off points are separate, specifically to avoid this kind of switching uncertainty and the possibility of false switching when things vibrate. Just what I'm seeing. I think that some of my switches are better than others so I shall go back and test more carefully. Anyone know a source of good examples of these switches? My ebay source doesn't seem to be there any more. I wonder why?

Please keep comments coming. The silliest problems are always the ones you can't see yourself