Ok, so I had this problem where my IPhone 7 would keep restarting while it was in recovery mode or it would simply not turn on after downloading firmware was complete. I suspected the issue to be with the battery, because the phone used to work perfectly and I hadnt known of a battery change ever, so I got a new battery from ifixit, popped it in, and went to restore the phone in itunes. The process went smoothly, and I had no issues. As the batteries get older they start to degrade and affects the IPhones functionality. Most issues like slow IPhone or random shutoffs, even itunes errors can be fixed with a battery replacement and its $30, so why not try it. If a battery replacement doesn’t help you with your issues, then your problem is on the logic board itself, either on the memory chip or somewhere else. For that issue, I would recommend looking up REWA technology on youtube and see what they do and try contacting them if they may be able to help because I know they have professional grade equipment where they are able to re-flash the IPhone’s nand memory, but im not too sure about that. Anyway, try a battery replacement and if it works, great, if not, we got a different problem on our hands. Good luck on your issue!!