I only had that problem with a rogue app that wasn't completely closing and would cause my phone yo lock up. Uninstalled the app since I didn't want it and haven't had lock ups since. Check last 1 or 2 apps you recently installed.
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I would also bet that it is a rouge app. I tend to install tons of apps that I only use one or twice and then never again because I lose interest or even just forget about them. As a result, I perform a factory reset every few months or so just to clear everything out and start anew. I then only install apps as I need them.
I know some would view this as being a PITA but I like starting out with a new phone every few months. I will probably perform my next factory reset once KitKat is available.
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