Other possible clues? :
1. Sometimes the 8SA relay responds immediately to a change from the ISY. But other times (more than I’d like to see), the response is delayed for many seconds. I can queue up several on/off changes and they are all delayed, coming out in a quick burst several seconds later. I can’t isolate when this happens, but it’s like the ISY is busy somehow. I always see this delay when I first restart the ISY.
2. I fired up the Event Viewer from the ISY for the first time. I see a lot of X10 traffic, but not constantly. I have a lot more X10 devices in the house, and defined in the ISY, than Insteon. When I do the Query on the main ISY folder, I get a burst of X10 status requests (probably for every X10 devices I’ve told the ISY about), then there is an X10-RSP 02 53 xx xx xx (no X10 device code listed) that goes out every second, repeated for approximately one minute.