Hey Atroz,
I should be in bed already but saw your post which of course makes me think.
Can you measure constantly the ‘logical high’ voltage on the probe, especially in the moment when the machine wants to move towards the probe and the erratic probe finding happens? Preferably with an analog multimeter (pointer instrument) if you have one, since with a digital it’s not sure if you can see very short variations of voltage.
Another thought is, is the behaviour the same if you remove the touch probe cable from controller and touch probe, and you wire the touch probe to pin 22 of the 25-pin I/O port?
Good night!