At Union Station in Chicago, they have quite a few recessed cans with HID lamps.  They're all coated, but I'm guessing they're MH and not MV.  Maybe 1/3 or 1/4 of the fixtures have obviously flickering lamps.  Are these most likely bad Metal Halide lamps?  It's been a problem for years, maybe decades, and you'd think they would just re-lamp them if the solution was that simple.  I've seen flashing MHs elsewhere, but the problem is unusually bad at Union Station.  In contrast, the fluorescent and incandescent fixtures are all very well maintained.

I saw some canopy lights at a car wash doing that.  They would actually flicker for a while, go out, than cycle back on.  As they warmed up, as they approached full brightness, they would start flickering again.  After several minutes they would go out and the cycle would start over again.

I periodically visit a factory in east Michigan to service a machine and the building is lit by hundreds of 400w MH high bay fixtures. Over the last couple years I've noticed that virtually all of the lamps have a swirling arc...the shadows around all the fixtures are rotating or gyrating. The last time I was there, however, very few of them were still gyrating so I suspect they were relamped. I tend to believe that MH lamps that act strange are a result of the bulb quality more times than not. Other cases I've seen such as sudden dimming and cycling were caused by capacitor degradation; and cap replacement promptly fixed the problem.

If any HID lamp continuously flickers with the mains, this may because of rectification. Because HID lamps can also rectify AC to DC like fluorescents and LPSs. This is why electronic ballasts for HID lamps also contains a DC protection like fluorescent electronic ballasts.

my school has about 45 MH recessed cans that are 50W most likely, and about half of the lamps are flickering/ cycling/ dead and the other half work okay.
the lamps seem to last about 5 years from what ive seen, they burn 8 hrs durring school year. as the lamps age the color shifts from pure white to a slightly dimmer bluish light,until about 4 yrs,  then they generally flicker for a year, then cycle relentlessly, then they fail completely, they may be CMH lamps. I doubt it since i saw one that turned pink one time. And a few of the ballasts or ignitors are dead due to cycling lamps.  magnetic MH ballasts for pulse start should have anti-cycling ignitors
i wish they maintained the lights better

one time they ran out of MH lamps and used HPS for a few months
i tried telling them about the problem but they didnt care much

My local shopping centre uses recessed MH fixtures (or MH-lamped MVs) in its older part. There was a "The Bay" that closed in early 00s, they converted the empty space and extended the mall. In that new part, huge CFLs are used.

Anyway, some MHs in the rest of the mall act like mentioned before. The arc swirls in the arctube, making a funny moving shadow on the floor! They never cycle though. I've seen that phenomenon in some other shopping centres with similar lighting configuration. I still can't figure out what's going on inside the lamp? Maybe the arc is seeking a spot on the electrode with less resistivity or more emissive substance and can't happen to find one?

I thihk that the problem with swirling arc in MH lamps is unique to american lamps and related to the ballast, since i have encountered this problem in Israel only once, in one of the shops of our local Office Depot chain of stores, in Grand Canyon mall. The lamp that encourted the problem was a US GE 400W probestart MH with phosphor coating lamp that operated on Eltam's CWA ballast. I never encountered this phenomenon with the european MH lamps that operated with an european style reactor ballasts.