I've noticed over the last couple of weeks, I've had a few moments where the phone seems to crash and freeze up. It's not happening all the time, maybe once every two or three days.
It always happens when the screen is off and the phone is locked (in my pocket, on my desk or on charge). I'm not 100% but it seems to be when the phone is being left unused for a longer period of time. I try to wake the phone when I come to use it and pressing either the home button or the lock button does nothing. The only way to restart the phone is to hold the lock button and do the manual reboot.
From the battery stats, I can see that the phone is still using power when it crashes, though it doesn't appear to be turned on and monitoring the battery. I get a huge step in the graph where the percentage drops and I can usually work out when the phone has crashed by knowing when I unplugged it in the morning and how long the battery says the phone has been on for.
I'm wondering if there's any sort of app I can use as a sort of diagnostic tool, to see what if anything is causing this. If it's an app that's causing a problem I can easily get rid of it and try again.