Is there any reason as to why this phone seemingly randomly reboots for no reason?
I'm going to keep posting this seeing as a handful of people mentioned it in this thread and EVERY person was completely ignored
It was probably ignored because no one has had an answer to it.
Never had that problem with my 635, however, I had that problem with my 520. I traced it to my microSD card. I had a bad one. When I moved everything from Card to Phone, then removed the card, everything started working and it went a week without rebooting. I put the card back in, moved a couple of non-essential apps to it, and started saving pics to it again. It started misbehaving that day. Got a new SD card, and never had the problem since.
The SD card in question never showed any errors when checking for errors on my Desktop machine. I worked with the cards maker for nearly 6 weeks troubleshooting the issue, because they were going to replace it under warranty, but the second card they sent had the same issues. They finally determined it was a batch of cards (both cards I tried had the same batch number, and were off by "6" on the serial number) that had bad controllers. The memory was fine, it was the memory controller that was bad. They gave me an upgraded card for the trouble, so I went from a 10MB/s read/write to 40MB/s read and 25MB/s write, which is okay for me, considering I had only paid $23 for a 64GB card in the first place.
Try using the phone without an SD card and see if the issue goes away. If not, then you may have a defective RAM controller or defective RAM in the phone, and your phone will need to be replaced. It happens. Could also be a defective CPU.
Try the SD card test first, as it will keep you from going without a phone. If that fixes it, then great, just get a new card. If not, then get a new phone, preferably under warranty.
Let us know how it turns out.