I know the post is old but today i found my solution for random restarts of my old P6, in my case that was vibration motor i don't know why (short or something?) when I disconected it everything back to normal, maybe someone will have the same problem
Before that plenty custom firmware etc ocfourse and nothing works but this works perfect for now
Update:
I've changed vibration motor and vibration works fine and phone also, no restarts at all