Well, mine (Lumia 920) has reboot problem, many times per day. It doesn't need any apps running, nothing. It just reboots randomly. I sent mine to service, only thing they did was install the portico update (took two weeks). First day I took the phone back in use, three reboots. Next day took the phone back to service. Now there is note with phone that sw update is not a fix, change something or send new phone in return.
So at least for me (and also for people who suffer severe rebooting problem) the portico update didn't solve anything. Its hw problem.