I have found...as with any computer...that when memory runs critically low
the system will freeze or shut down. My Razr M has done this 3 or 4 times on me and I have noticed that it has done so when I have pushed the storage limits to the hilt.
I only have an 8GB storage. Besides the onboard apps and such I have downloaded up to 3 full sized movie files, almost maxing the storage capacity. The reboots come during playback of the movie.
This does not happen with only two movies on board, meaning I have about 1.2GB of space left. I've downloaded several apps and had zero problems with either app. The problems only start when I push the storage limits. I figure a 16 or 32GB SD card will help solve this problem.
Anyone else having these issues should go into their system settings and see how their storage capacities are doing. It is a good idea also to make it a practice of flushing the caches on the apps, thus freeing the system of unnecessary clutter. Check under files>Device storage also. You may find alot of junk files here that may have been downloaded via torrents or direct downloads.
Dump them. Dump the browse history on Chrome regularly also.
My Motorola Razr M is hands down the best smart phone I've ever used. No problems a regular techie can't handle.