is not the battery, IT reboots!
is not a lose battery because this phone, different from lot of other phones, the battery does not just fit with pressure... it has two clamps that needs to be locked in order to hold the battery there EVEN in the event of a really big drop and so well designed that the battery door does not close until both clamps are locked.
Once trying to take a video while riding a bike i hit a bump and the phone fall to the street, almost 20 or 25 vehicles passed over it until there was a gap in the transit so could pick it up.. the dam phone is so tough that the phone recorded EVERY vehicle that passed over it
regarding cleaning contacts... neither, it does it with the phone new, coming out of the box