Many thanks for the response, Walter.
1) Glad to hear the "Inhibit Options" is a beta thing. Don't often go to that page, so wasn't sure how long it'd been labeled that way.
2) My 720's OS version is 1.0.1.9
3,6) I understand re: weather radar delay, and was factoring that in accordingly. Several times during my flight I flew through "green" on the display and was in dry skies, or "clear" on the display and was in good rainshowers. But when you see a blob of stuff on the radar to the right, and can look out the window and see corresponding ugliness to the right, it helps give you the warm fuzzy that you're seeing the story pretty well. And if you can't see it, then the radar may be the only thing that you've got to try to keep a wide berth around the embedded stuff.
That's why it was disturbing to see the image "jitter". This wasn't simple lag/delay, this was "it's in front of me, no, it's behind me, no it's in front of me, no it's behind me". I was only glancing down at the screen occasionally, so I can't give you hard data about how long the different drawings persisted, but it was at least tens of seconds each time (so it was extremely low-frequency "jitter"!). As a guess, I might say that it drew the image correctly (in front of me) for 2-4 minutes, then drew it wrong (behind me) for 1-2 minutes, etc. This cycled at least three times. I'm not sure exactly how big the shift was in distance...maybe ~10 miles or so? Big enough that it was confusing/disturbing the first time it happened. "Hey, what's going on, how did I move so far so quickly?" Then I happened to be looking at the screen when the image got redrawn with one of those big shifts and I thought, "Oh, okay, this is a bug of some sort."
4) Ah, thank you for the reminder that the Mode>Weather label gives a clue into ADSB data reception status. That should have been where I focused my attention to get the big picture of what was causing my missing data. Thanks for the reminder.
5) I was using whatever beta had been released as of last Thursday, but I don't remember which one that was. At any rate, I never saw anything in the large window on the Connected Devices screen during my troubleshooting. The next time I'm in the plane and sure I'm getting ADSB data, I'll look at that window again and see what it's supposed to look like.