My PBP starts up like a 1977 Chevy Nova

[rotwang]

I'm outta ideas here. It crashes too much to be usable. I first received this thing in April and I have been poking at it since I got it, but it has never worked.

This level of frustration is likely to put you off from entire companies or perhaps even categories of products (SBC in general)?  I applaud you for sticking with it.  OTOH, if you do ever get it working, you will obtain a (well deserved, IMO) sense of accomplishment. 

Could the emmc be bad? That seems unlikely to me because I dd doesn't throw any errors when I write an image to it with the usb adapter.

I would check, again just in the name of thorough troubleshooting, especially since it seems you have tried everything else.  If you don't want to spend the money/time to purchase another, and are willing to share your location, maybe (slim chance) someone can meet you to help (if they have a spare)?

Above is one big reason to become friendly with local hacker spaces, LUGs, HAM clubs, etc...

Alright... so I'm not _really_ out of here. I was just being dramatic.

If I were seeing errors or something when I was writing the emmc with the USB adapter, I'd buy a new one in a second. But it seems to write fine, which makes me not much inclined to shell out for a new one that isn't likely to solve my problem.

I'm in NYC -- if anyone reading this is in the area and wants to loan me an emmc to try booting, there's a coffee in it for ya!

As for my sense of accomplishment -- I think it will be severely diminished if I have to buy a new $100 mainboard for a machine that never worked! You might be right about the category of products for me. I'm not really super into buying lots of hardware, Making old reliable hardware work years past its life expectancy is more my thing than getting my hands on lots of iterations of new experimental stuff. The long life expectancy and availability of spare parts is what got me excited about pine in the first place!