This is an 18T?
Does it happen often enough that you can watch the ladder while it runs?
(I'm guessing "not")
Could be that it is seeing the Feed Hold button even tho you are not pressing it?
If you find the Feed Hold button input in the ladder, see if there is any flicker at all.
I don't know the weather in NJ, but we are just now seeing short sleeve weather here.
I have started to add an air conditioner in the summer to keep a ghost out of my lathe.
On the 51, I removed the cheesy cooler that is on the back door and mounted a real A/C there, but that alone is not enough as the control is actually in a different hole above the spindle. So I popped some holes below and to the right of the RS232 / 120V AC out let cavity, and mounted a 4" 'putor box fan over it (drawing air OUT) and plug it into the outlet right there. This will pull fresh cool air through the cavity where the control lives.
[need to have a fresh air inlet on the opposite side tho to allow air flow through]
If you can't find a logical fix otherwise, this is worth a shot.
I would actually unhook the red button and try it like that first tho.
Doo to the fact that it happens with different tools at different places in the cycle, AND that the spindle stays running - I'm pretty sure that it is NOT a raw wire in the loom.
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Think Snow Eh!
Ox