Don't really know what's causing it, but today I pulled my phone out of my pocket and it was totally unresponsive. Pressing the power button did nothing. I had to hold the volume down + power to get it to boot up again. I don't have many apps, but I uninstalled Nokia Drive Beta just in case since that one seems to be causing some problems.
I haven't even had the phone for 48 hours yet, so this is very disconcerting...
After that happened, the phone got really warm and the battery seemed to be discharging faster than normal, so I rebooted it again. Not quite as warm now. Still concerned about the lockup, though.