Don't know if this is related to your problem or not but from time to time in the past I've had e-stops when the cutting loads suddenly increased. If the cutter was clogging up (aluminium) and slowing or stopping the spindle it could trip the e-stop.

I assumed this was increased electrical noise from the VFD as the amps shot up, and any long cables were like antennas which fed this into the BOB. In particular the long run to the Z seem most affected.

Since upgrading to 24V on the e-stop & limit circuit (i.e. relay into the BOB with very short wires) I've not had an unintended e-stop due to sudden load increases.

So when you do an air cut things are different to doing the real cut in terms of VFD load and this is another factor to consider in the diagnosis. Having said that if Mach3 is not e-stopping then it surely points to something in the UC300 or USB?
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Not sure if you were joking but this forum was set up by Lee Roberts [and we are all indebted to him for doing so]. Dean (JazzCNC) is a massive contributor so you could be mistaken for thinking that, but it is not his creation. On that subject I think Jonathan is still a moderator ?