I have had something similar happen on certain tools, and all of them had coolant through pull studs. When I would take those tools out of rotation, the problem would go away. Use them and it would randomly come back. My machine is an early model without the extra air tank on the back (hmm, I wonder why they added that?), and it is sensitive to low air pressure on tool changes, and a hollow stud while it is ejecting a tool is sometimes enough to hang the change.