This time I replaced the return spring, and cleaned up the slide again. This time I noticed a couple of small burrs on the slide, and I have no idea how they got there. A little steel wool and they were all smoothed out. I also adjusted the actuator rod a little, ran a quick test of 100 rounds of 9mm, and the machine operated flawlessly, so I think it's all good, with no need to order a new primer slide.
Thanks to all who offered their assistance, it is greatly appreciated.