So a couple of days ago, I saw a 175w mercury vapor lamp that was exhibiting what appeared to be a 30hz flicker. Now, the lamp appeared to be brand new, and the arc didn't appear to be moving at all, so now I have to ask, why was this lamp flickering the way it was.

Was it rwally 30Hz or rather 60 (normally running lamp has 120Hz flicker, not 60Hz as many people use to call it)?

Otherwise it may be the arc root "dancing" around on the cathode, so from one side it may seem as a flicker. That way the flicker could be any subharmonic of the 60Hz or even rather random.

I thought it was the arc dancing around, as I've seen the same things more than enough times with MH and Fluorescent lamps, however, I could clearly see the arc in this lamp, and it appeared to be stationary, which is what really started to confuse me.

Also, I'm pretty sure it was 30hz, as I could stand perfectly still and still see the flicker, versus 60 or 120hz flicker, where I have to be moving my eyes to see it.

Probably has a gas fill issue or is slightly out of spec. Usually this happens temporarely with new lamps though from what i noticed

Probably it was a strong 60hz, as a result of rectification. This can something occur with MV and probe-start MH lamps.

... however, I could clearly see the arc in this lamp, and it appeared to be stationary, which is what really started to confuse me.

I think your eyes could easily play tricks on you with this.
The flicker is practically visible by peripheral vision only, but the exact shapes by the central vision.
So once you focus on the arc, the arc comes to thecentral vision so you don't see that to flicker. But of course you still see something flickering, the eyes just trick you to believe it is not the arc.

The arc always has only one "root" on the electrode. So if you "see" it "forking" into multiple roots, in reality the arc is dancing (each period it get ignited from the different root point) and that is then the source of the subharmonic (fraction of the main frequemcy) flicker. The "forked root" wn't appear to be flickering because it is in your central and not periphery vision.

I guess that makes sense, since I've experienced this with some fluorescent lamps (where they look fine when looking directly at them, but appear flickery in the corner of my eye). Anyway, thank you for your insight, Medved.

By the way those vision effects are the reason for the existence of "flicker meters" (a deviice indicating low frequency AC component of the light intensity, often with narrow angle focus so you may work from the ground) as a real service/maintenance tool. Because when you are hunting for the one bad flickering bulb in a group, eyes are really betraying you, with the flicker meter you can quickly identify the offending one...