Just found this thread...I have been having frequent reboots/resprings on my 6 plus for a while now though I can't say exactly when it started. Happens probably 4 or 5 times a week, often while updating or switching between apps. I took it to the Genius Bar a couple weeks ago and when he scanned the phone he was able to see the crashes in their diagnostic software. He did NOT offer a replacement however as he said the operating system was somehow corrupted and the only solution was a DFU restore (without a restoring from a backup) in which I would lose all of my saved text messages and I would have to reinstall all my apps from scratch. He did suggest however that iOS 9 might fix the problem as theoretically whatever file is corrupted would be replaced, but he was not positive about this. I did not want to lose my messages and go through all that work reinstalling apps so I did try a DFU restore followed by a restore from backup just for kicks and I am still getting the crashes.
I find it funny that the Genius people say they can see the crashes in the logs, yet the Apple engineers say they can't see it.
For what it's worth, the "Genius Bar" people usually don't have much of a clue about software issues unless they've gotten an edict from HQ. Their suggestion is almost always a restore and set up as new. Case in point, my 5s was frequently kernel panicking when charging. Based on the error code it appeared the flash was bad. I brought it into the bar and originally they said I should restore it when I described the problem until I showed them the logs. They were told to do a hardware swap for kernel panics, but they told me to set it up as new or else the kernel panics would come back. I did a restore from backup and it's been fine ever since (nearly 16 months).
iOS 9 is supposed to be a stability release, but from what I've read it's fairly buggy, similar to every x.0 release the past few years, so even if it fixes this problem, it's bound to introduce others.
