Hey guys,
I am not owning an FP2, but a colleague of mine does. He demonstrated once that he can cause reboots by clapping the phone with his hand. To me this looks like a loose/unreliable power contact rather than a software issue. Maybe it’s just the battery not sitting tight in the case? For some reason I haven’t heard about this trouble with FP1 (at least another colleague of mine using FP1 does not have this issue).
You could try to narrow this down by avoiding shocks to the phone (i.e. having it stay put on a table?) and observe whether it’s rebooting as usual. The next step then would be to identify the loose component, which might be much more nasty.
EDIT: Sorry, as this was discussed multiple times already in this forum, I guess you already ruled this out. Didnt want to duplicate.