Ahh, now that would indeed explain it. The Radius greyscale that worked "perfectly" was way across the room - too heavy at 70lbs or so to lift across for the sake of a brief test, so I just plugged in an extension cable.

It has been a long time since I read about RFI and CRT monitors in the old setup manuals. This is the first time I have actually encountered it.

I will experiment some more with screens and distances, but am cramped for space because of all the other hardware lying about.