Re: Metal Halide arc instability...

Mabye I should reword this, then mabye somebody will answer...

I've noticed that some 400w metal halide lamps of different brands
suffer from undulating arcs(arc within arc tube moves back/forth) only
in some buildings, whether the lamps are new or not.

What would cause undulating arcs in new and old 400w metal halide
lamps?
Why is this problem localized only to certain buildings?  Ballast
type?  Capacitor value?  Power line problems?