find_orphans.py found (and was able to remove) a bunch of snapshots. It found but could not remove two orphan recordings. Removed those as root, some sort of ownership or file protection issue. There were some zero length ts files, which it removed. Finally it found 5 or so recordings for which there was no video. (Perhaps these were the same as the zero length files, I did not check.) It did not have a way of removing those so they were removed from within MythTV. Except for one "football night in america", which had no entry that I could find in the list of watchable recorded shows. Is there some other way to find out, other than through "watch recordings"? Maybe it is liveTV - we don't use that intentionally but might have triggered it once by accident.
The power supply has been replaced. We will see if it makes any difference. I also opened up the case to clean it out. Except it wasn't at all dirty. Just a tiny film of dust near the back vents on the bottom of the case. The circuit board was clean. The tuners were pretty toasty though. Nothing was being recorded, nothing had been recorded in at least the preceding ten minutes. Does the HDHomeRun have a way of turning off the tuners when they are not active, and if so, does MythTV do that?
Been a week with the new power supply and no bad recordings other than from weather. So apparently that was it. Now I just have to remember how to mark a thread as "solved".