RPI3 sd lockups?

[Greg Troxel <gdt%lexort.com@localhost>]

I have a few RPI3s, all running netbsd-8.  Two have been entirely
stable, one on a UPS and one not.  One, not on a UPS, has over the last
month or two started having odd lockups.  The symptom is

  logging messages that the sd card cannot be written, but not medium
  error.  Looks like controller out to lunch.  I don't have the text -
  it doesn't get writtten.

  weewx ends up with multiple copies

  kernel accepts writes to files, but the blocks don't make it out

  on inspection, the sd card is fine


On my todo list is updating to netbsd-9.  Besides that, I wonder if
anybody has wisdom in terms of:

  - smells like power supply issue
  - smells like hardware failure
  - known netbsd-8 issue
  - smells like SD card issue anyway

?