> >Any expert in neon physics here?
>
>   Not me. Maybe Van Horn will chime in.
>


Who me?  :)

I'd guess that the photons are interacting with the neon atoms along with
the electric current. The neon glows when it relaxes from an excited state,
and in doing so, emits a photon. It can also absorb a photon and enter an
excited state..

I've seen the effect he mentions, but I don't know the cause. It seems more
prevalent in older lamps, so it would seem to be amplified by some change
that happens in the lamps over time.