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Very interesting. I am also having same problem on Z1. It all started about two months ago when i updated facebook. I thought it was only coincidence. Sjnce then i have resetted my phone to factory settings uncountable number of times but the problem refuses to go. Now my phone just freezes whenever it wants and i have to do a cold reboot everytime. Its kind annoying that o havent gotten a solution. All that is being posted here are just suggestion but no tested and proven solution. Not even an indication on what exactly it is that is going on wrong. I wonder whether Sony ever read these complaints and what their research team is doing about it.
I have a Xperia Z1. I had to uninstall the Facebook app, and that was the solution!!!!
To have FB I have installed Faceebook Lite app, but I don't like it, it's too basic!!!
To solve your phone problem, you have to uninstall the Facebook app. I hope that Sony will repair this issue someday!!