I had this exact problem starting one week after getting my 3rd gen SE (second gen never had this problem). After the second shutdown, knowing it wasn't a one off, I called Apple support. I was told, initially, that it was software related, but the tech wanted to run the analytics logs by the engineers just to be sure. Well, a few days later, and I get the call from Apple telling me that they believe it's a hardware issue, and they need to swap out the phone. And, they want me to keep everything on the phone, but removing any personally identifying info. So, if it's hardware related, why do they want all of my apps left on it? Honestly, I don't think anyone at Apple truly knows what's going on, as they have told me that this is absolutely the first they've heard of a SE model having this issue (it's always the first, isn't it?). Now, my advice to anyone experiencing this is to get support to acknowledge the problem (if your analytics logs have a "panic" node in the lines of text, that's it), then get them to agree to a full refund/replacement, Don't let them screw you around; there's something going on with these phones, and the only way to get them working on the issue is to start demanding refunds.