Wagner asked:

  (detailed recounting of personal neon experience skipped)

>Any logical explanation?  What could be the effect of photons in neon
>gas?
>
>Any expert in neon physics here?

  Not me. Maybe Van Horn will chime in.

  I do remember that when I first got into the electronics game,
  RCA was the top dog in the vacuum tube game. They also published
  individual catalogs of their special-purpose industrial and
  scientific tubes, including phototubes.
  They made a "gas" phototube which had a highly attenuated fill
  of noble gas. It was operated with a very large load resistor,
  many megohms, and a DC supply voltage of about 50-90 volts.
  This limits the current to a level too small to sustain a
  continuous discharge. The principle is that photoelectrons
  traveling to the anode collide with gas molecules and create
  positive ions that add to the photocurrent. These tubes were
  popular IIRC for reading the optical sound track on movie film.

  Neon lamps were once used as DC voltage regulators. One reason
  that they were not very good ones is that, as Wagner discovered,
  the breakdown voltage could be influenced by many things
  including the photoelectric effect. To stabilize it,
  manufacturers put some low-level radioactive material in
  the envelope. This generated enough background ionization to
  swamp that effect out.

  Try putting a magnet next to the flickering neon and notice
  the effect.

  Reg Neale