Picked up a lamp post from up the road. It is an American Electric Lighting 245 post top with a 150 watt HPS mogul base. Once powered on the lamp will get to a pretty high brightness. After about a minute the lamp will go out. During the next few minutes, I can see tiny lightning bolts jumping throughout the inside of the arc tube. This bulb was already in the fixture when I got it so I have no idea how old it is. I’ll provide pictures later. The arc tube is a light gray with dark gray marks at each end of the tube by about a quarter inch or so and a few small black specks though out. The envelope doesn’t appear blacked at all. I am wondering if this lamp is just at the end of its life or if something is wrong with the fixture. Thanks!

The behavior from the lamp that you described is pretty typical behavior of HPS lamps at EOL. Your light is fine. All you need is a new lamp and your light will be shining bright.

Yeah, it's typically known as cycling in the industry. A new lamp will fix it right up. If you let a fixture run with a cycling lamp for too long (like weeks/months) , the ignitor can get strained with all the starts it has to do.

It's the reason why HPS streetlights seemingly turn off and on at random sometimes.

So I compared the lamp from my post with another sodium vapor lamp with a lot of hours on it. Both tubes look the same. I have attached a photo of the arc tube from the post lamp.

 Your HPS lamp is EOL .   Just a new S55 150 watt HPS will fix the problem. 

Are 150 watt HPS mogul base bulbs not as popular? It seems like a lot of places both here and online for hat carry 150 watt medium base HPS. I found what I’m after on Amazon but everywhere local only has them with the medium base.

Mogual base HPS is very common.  Esp with utilties
And some HPS yard blaster use them.  Thing is lot of
med base HPS usually found in residental grade Hid
 Fixture.  Go to a dedicated lamp shop that sells bulbs
Or electrical supply house and farm supply stores.  Big
Box stores are getting some what limited as they are
Trying to push out HID lighting.

You'd be able to find plenty on eBay as well, including some older USA-made lamps if you prefer that.