Recently, there were couple of people complaining about restarts which later turned out to be unsecured and infected devices: http://forum.mikrotik.com/t/router-compromised/121685/1
If it is load-related, you should see the issue in /system resources
If it is hardware related, you should see autosupout.rif which you can send to Mikrotik Support email and they might figure out why is it crashing.
Also you may connect computer to the serial port on your CCR and watch the output during boot. there might be also some info about the crash.