I believe I've confirmed that the problem was my gimbal guard that I foolishly installed when it came unexpectedly with my landing gear extensions, and which blocks the downward sensors. With the guard off, the aircraft performed well/normally/perfectly? on 4 successive flights (the third one with the VPS turned off as well). Am including the logs if anyone cares to look at them.
DJI Flight Log Viewer - PhantomHelp.com (just hovering, then up slightly above the VPS effective height, then RTH)
DJI Flight Log Viewer - PhantomHelp.com (similar, but with more manoevering and forward flight, considerably higher, then RTH)...
DJI Flight Log Viewer - PhantomHelp.com (VPS off, with RTH)
DJI Flight Log Viewer - PhantomHelp.com (VPS on, manual take off and landing; mostly testing camera/image resolution at specific altitudes)
So, I apologize for taking up your time when the problem was primarily my own stupidity/ignorance. I would also like to express my gratitude for the helpful attitude displayed by several of you, and for the sincere and sustained efforts to help out a rookie.
While I don't plan to bury the forum in questions, this experience has given me confidence that the forum can be a place to seek assistance without being concerned about hostility or impatience like I often see on some other forums. And, I've learned a lot in the course of troubleshooting this issue and reading your suggestions.
Thanks,
Phil