[Galaxy Note5 Q&A, Help & Troubleshooting] Crike:
This is a running journal covering my attempts to stabilize a secondhand Note 5 prone to crashing. This first post summarizes the progress, and links to the current state of affairs below.

I've just acquired a second hand Note 5. It was sold as problematic, restarting continuously after loading the stock Marshmallow. 20 seconds or so, and it reboots.

After performing a factory reset the first time, Marshmallow loaded the configuration interface without any issue at all. Select region/language, choose WiFi network, give password, choose email account, give password, etc. Eventually it loaded into Marshmallow itself, no problem. It worked fine for half an hour or more. Then it froze and rebooted, during the automatic (app) updating process. After that, the screen would freeze after 20 seconds or so.

I tried another factory reset. This time it froze and restarted twice during the configuration interface, right when entering the WiFi password. After I got through it, and eventually loaded into Marshmallow, I turned off WiFi. It has been working without issue for an hour now.

I noticed a "dm-verity error" message at the bottom of the Marshmallow stock recovery screen. Perhaps importantly, eventually I recall that on the download screen the image and binary are identified as custom, not official.

See post #3 for more details.

By post #14 the phone has stabilized somewhat. Both WiFi and cellular function and the sudden restarts have been much less frequent. I successfully flash the Russian (SER) Nougat (7.0) firmware of November 2017. The restarts end completely, the battery performance is spectacular (1% per hour on idle) but l lose the cellular network.

By post #19 I suspect either a hardware or persistent data issue is involved, and find with *#0011# that the IMEI Status is NG. I am considering how to unbork the situation, as I didn't make an EFS backup.

By post #23 I have restored network service, but no matter how thoroughly I wipe and flash stock firmwares I'm left with the original issue: constant freezes and restarts. Then, without any discernable reason everything is smooth for days. This odd behavior leaves me wondering whether it's not a hardware issue, but the previous owner borked partitions that aren't affected by flashing firmware. I decide to try and make a full backup, including all hidden partitions normally not included. And see what happens if the next time the phone starts crashing, I restore that backup. But how do I make and restore a full EMMC backup?

It turns out the internal storage of a Note 5 is actually UFS, not EMMC and XDA turned out to be a dry well on full (EMMC) backup and if Google had answers, persistent searching for a week or so didn't turn anything up. Android backup seems an ill defined and ill standardized field. Many options, many superficial. What imaging there is, is spotty and generally incomplete, "proprietary" and not easily restorable.

When after a month smooth operation things go pear-shaped again (post #24) I've got no backup to restore. Not that it would have mattered probably. I suspect it's probably a NAND/EMMC/UFS issue at the back of it all. Now, with Note 5 not officially marketed or supported in the EU, how do I find the parts that I need and fix them?

In the end I replaced the motherboard with one I bought on eBay. The process was simple, using a YouTube video to walk me through the steps. I did walk into a peculiar circuit sticker which was partly stuck to the top of the inside of the cover, and tore. The switch out worked without a hitch but I'm trying to figure out what the sticker is. See my last post.