Metal Halide arc instability...

Back when I was in middle school I noticed some of the MH lamps in the
gym had unstable arcs (arc moving back and forth within arc tube 3
cycles/sec).  The lamps were standard 400w metal halide in Cooper high
bay fixtures.  The lamps were aged considerably, and the fixtures in
question were casting some light on the ceiling above, indicating the
arc instability(as well as darkened arc tubes).  I thought the lamps
were doing this because they were old.

However, my family took me to Hawaii a couple years ago, and every
single 400w MH lamp in the airport had dancing arcs (again, arc moving
back and forth within arc tube 3 cycles/sec), and the lamps in
question were in good condition.

Recently, a Costco was put in near where I work.  The lighting in
there is high bay fixtures, unknown manafacturer, 400w MH.  The lamps
were new, and about half of them have the dancing arc phenomenon
(again, arc moving back and forth within arc tube 3 cycles/sec).

I've noticed the dancing arc phenomenon mostly in older 400w MH lamps
in stores, etc.  but this phenomenon manifests itself only in some
stores and not others.

I've noticed the above with Sylvania and Philips 400w MH lamps, but I
can't seem to reproduce the phenomenon with a Philips clear, Philips
coated or GE clear 400w MH lamp hooked to a Sylvania 400w MH ballast
with a 24uf cap running off 120v./60Hz

What would cause this kind of arc instability only in certain
inatallations of old and new 400w MH lamps?