Found the culprit! The little bracket on the stepper motor is made of injection molded plastic, with brass threaded inserts where the bolts go. The plastic around the threaded inserts were cracked, both top and bottom. The bottom was broken completely off, and the top was just hanging by a thread. Either it broke when I was cleaning the nozzles, or perhaps the screws were over-torqued from the factory. In any case, I fixed it by machining a new bracket from 6061 bar stock. Problem solved. Took some pictures but I don't know how to upload them.