Disconnected the X and Y motors from their X-Y table. Unable to reproduce the problem of horrible sounds and missed steps described above when MACH3 jogging the motors at full speed. I'm becoming convinced that, since the X and Y-axis move very smoothly on their own, that the issue - in my case - is related to some resultant force vector in the heavy X-Y table causing table twisting and thereby increased friction and too high loading on one or both of the the motors (even though motor currents seem low). I can't tell by sound or feel whether it's the X or Y motor generating the noise. A mystery that's annoying and puzzling.