I've also had this problem in the past. From my experience, it would most often occur when the battery was below 15% for some reason. Never could figure it out, and it has not happened in a few weeks. I do seem to have a similar problem where the phone seems to freeze for a few minutes and I get the samsung screen, but it does not power down like before. I believe this to be a problem with available memory. Either way, I never found a proper solution and wrote it off as a phone that can't handle its own capabilities. My next step it to uncheck the "prevent force close" features on some of the apps such as
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