Update...ShopBot tech support had the clever idea to run a job with the DC running but with the flex hose removed from the dust shoe (I simply draped it over the ductwork near the ceiling). Two jobs cut without incident like that. So the issue appears to be static in the flex hose itself, which obviously isn't draining sufficiently despite the bare copper wire running through the hose and attaching to the metal ductwork at the top.
Then an experienced electrician came to the shop and looked at my setup. He immediately said that it's not sufficient to run a bare copper wire through the hose and then attach it to the ductwork, and then let the ductwork be the "raceway" for the static until it reaches the DC. Instead, he said the bare copper wire and the flex hose wire, which are bonded at the top of the flex hose, should connect at the top of the flex hose to regular #10 insulated wire, which is then tacked to the ductwork with circle terminals at the beginning and end of each section of ductwork. That way, the ductwork and the ground wire become one and run all the way to the DC itself without any opportunity for interference. And finally the #10 wire continues directly from the DC to an open terminal on the ground bus of the electrical panel. All together, he said that will create a tighter pathway to drain static without relying on metal ductwork and electrical conduit.
So that's tomorrow's plan...