Thanks for the suggestions, folks, and the humor. In this hobby, you have to smile sometimes when things go from bad to weird. I have tried a few different things, including inserting an old JVC turntable and Audio Technica cartridge into the system just to see if the popping would continue. It did. Now I am convinced the problem is in the phono stage. There is a kit included with the PH3 which, when installed, will lower the gain for higher output cartridges. I'll also call Audio Research for advice.
Of course, if anyone around here as any other ideas, I'll try most anything at this point, including an exorcism.