I have a 6200 that reboots. When it does happen, here's when:
It's been off all night.
Its alarm sounds, I snooze once, and allow it to turn itself on after the snooze wake-up.
I turn on its radio.
While listening, it makes a short buzz sound, I look at it, and it's rebooting. It might do that once more, but doesn't happen again that day.
This is apart from its inability to receive/make calls sometimes, even after several manual reboots.