How do your Magpul's hold up? I've used them for a couple years now, I usually have to order a new case ever 8-10 months because they start to stretch. Otherwise they are a fantastic case, slides out of the pocket but won't leave your hand. They are more of a guy case though, my wife hates it.
I started with an iPhone 2g and I can't remember ever seeing a phone crash to the point that it restarted. I'd check which apps are up when it happens, are any of them new enough to coincide with the start of this behavior? Maybe something that updated recently? If it were my phone I would try to identify a couple possible problem apps, delete them, make sure I have a full backup and factory restore it, reload your backup. I've done that a few times and it tends to clear space and improve the heath of the device each time I do it. To bad there are not really any diagnostic logs to look though.